I'm going to summarize everything now...

You need to have a military force that is big enough to defend your population, provide humanitarian and peacekeeping operations through your territory, and if that same force is also your scientific and exploration arm as well as your diplomatic arm when dealing with new species and attempting to get them to enter this Federation... That force is going to be huge.

Older Starships.

I've been looking at some real life military examples and some Star Trek examples.

Older Military ships still in commission.

The U.S.S Constitution, 212 years after her maiden voyage, this ship is still in active service with the US Navy, mostly for recruiting and public awareness tours, it does sail though.

The U.S.S Enterprise is another, at 49 years of age, scheduled for decommissioning in 2013, having been in active service for 51 years.

The U.S.S Jason served for 51 years.
The U.S.S Kitty Hawk served for 48 years.
The U.S.S Independence served for 39 years.
The U.S.S Mauna Kea served for 38 years.

As for Star Trek ships? I'll just focus on the Enterprise since it's the one most "history" is known.

U.S.S Enterprise NCC-1701 served for 40 years (2245-2285) before being destroyed.

U.S.S Enterprise NCC-1701-A served for 7 years (2286-2293) before being decommissioned... fan conjecture is that this ship was operating under a different name prior to 2286, probably as the U.S.S Yorktown.

U.S.S Enterprise NCC-1701-B served from 2293 to some time between 2315 and 2333 according to the novels and the fact that the next Enterprise was commissioned in 2333, because that would make the ship between 22 and 40 years old that it could have continued in service under a different name or may have been converted to a museum piece.

U.S.S Enterprise NCC-1701-C served for 11 years (2333-2344) when it was destroyed defending a Klingon colony from a Romulan attack.

U.S.S Enterprise NCC-1701-D served for 8 years (2363-2371) when it was destroyed by Riker being at the helm instead of Picard... I mean... Fighting off an attack by Klingon sisters Duras.

U.S.S Enterprise NCC-1701-E was commissioned in 2372 and was probably going to be named something other than Enterprise until the previous Enterprise was destroyed.

Seems to me that the US Navy has a better track record with ships named Enterprise... The Federation keeps having them blow up.

You can see that the first Enterprise with Kirk as Captain underwent some major upgrading... the ship is slightly larger and configured slightly different, but you can see that same sort of thing happening in real world ships when they are reconfigured for further service, like the HMS Audacity which was originally a German merchant ship named Hannover that was captured by the British and converted into an aircraft carrier. The U.S.S Midway which went through a series of upgrades from the old WWII style carriers to the more modern super carrier styling. Other ships have also been altered and converted into drastically different looking vessels.

Anyway, I'd think that because Starfleet operates primarily in space, that the ships would probably last longer as they're not dealing with corrosion. And if Kirk's Enterprise is any example they could keep swapping out components just like real world counterparts and extend the lives of the ships.

A ship may be in service from 2245-2379, but it might not look the same by the time it hits 134 years old.

Like the life cycle of a Galaxy class starship is supposed to be 100 years with 5 major shipwide system swapouts and upgrades at 20 year intervals, and minor refurbishments and upgrades every 1-5 years.

Now imagine a ship that is semi-organic(or organic like) with self regeneration abilities instead of a fully hard structure... Anyway, that's where I'm headed.

"Economic" concerns in the Federation

This one might be the longest yet.

Pattern replication technology:
Food, clothing, materials, equipment, toys, instruments... pretty much anything can be replicated.
You need matter to make matter still though, so recycling is 100%, there's no garbage dumps, everything can be dematerialized into it's most basic particles and reassembled into pretty much whatever you want.
There are restrictions on the technology, you can't produce poisons or weapons in your in home replicator unit.

This technology would have a negative impact on the farming community, as more and more people get their food from replicators, they become less dependent on farmed goods. By extension it would impact grocery stores, restaurants, and other businesses. There would also be an impact on manufacturing companies, they would be downsized to the people designing products and probably assembling prototypes, but otherwise the majority would be using replication technology.

Of course there would be niche markets... like for people that want organic food and hand made products, and those niche markets would be staffed by people who genuinely take an interest in that product... such as Sisko's Creole Kitchen run by Joseph Sisko who managed the restaurant because he loved to cook and loved people, and winemakers like the Picard family, I find it likely that Jean-Luc Picard or Marie Picard or both would have been given ownership of his families properties after the deaths of his brother and nephew.

And because people in this moneyless society did things to better themselves and do good for others, pretty much doing stuff to be happy... there would be cooks, farmers, etc... just not any in it for the money.

Don't you have to look into the stress and strain that active service puts on the frame of the ship?

How much stress do you get in a vacuum with structural integrity fields, inertial dampeners, artificial gravity and all that jazz operating at the same time?

If all that doesn't increase the lifespan of the ships you're going to be in trouble.

Plus in the case of the Enterprise they rebuilt the frame of the ship when they did the major upgrades between the end of TOS and the start of the Movies.

At the end of The Undiscovered Country, assuming the Enterprise-A was a renamed Yorktown about the same age as the Enterprise, they say they're to report to spacedock for decommissioning, Kirk later says the ship will be operating with a new crew, so I take that decommissioning to mean the decommissioning of the crew and not the ship.

Who can tell how long it was in service for after that if you ignore the novels.

The ship could have been renamed once again, transferred to auxiliary services, turned into a museum, traveled the Federation as a recruiting method for Starfleet, whatever.

I'd think the 100 year service time for the Galaxy class is not based on hull stresses and other stuff, but based on technological advances.

Building Starships

I don't think it unrealistic for the Federation to build ships the size of a Galaxy Class Starship on a planets surface... here's why...

The Federation has the ability to manipulate and generate gravity fields.
Energy is not a concern.

I also think that because of the gravity generation and manipulation of gravity fields that propulsion would be based on this technology as well...

Jobs in Starfleet

There would be lots of opportunities for various types of jobs in Starfleet, I'm going to use the US Military and on screen stuff for my examples of occupational areas within Starfleet.

Art and Design
Armored
Artillery
Civil Engineering
Clinical Care Providers
Combat Engineering
Computer Science
Construction
Cryptology
Dentists
Education
Electronics
Emergency, Fire & Rescue
Explosive Ordinance Disposal
Flight Operations
Flight Support
Health Care Administration
Health Care Sciences
Hospitality Services
Human Resources
Infantry
Information Technology
Information Warfare
Intelligence
Languages
Law and Legal Support
Law Enforcement and Security
Linguistics
Mechanical and Industrial Technology
Medical Support
Music
News and Media
Nurses
Office and Administrative Support
Physicians
Pilot
Public Affairs
Religion
Supply and Logistics
Systems Engineer
Tactical
Telecommunications
Transporter Technology
Warp Field Specialists
Weapons Technology

And that's just the short list, there's a whole buttload more because of sciences and specialties within fields... Obviously don't need all of these on any given ship all at once.

You can break down a lot of the occupations into major categories, a lot of them are cross category though... Like if you're a Weapons Technician in Starfleet that would likely be a cross between Security/Tactical and Engineering. A person involved in the Communications end of it all would be mixed up between Security/Tactical, Engineering, and Operations.

I really like the modern Military uniforms better than Starfleet, the TOS Movie uniforms used from STII - STVI I think are the closest they've done to something more realistic in my opinion.

There would be different uniforms for different purposes:
Combat Uniform: This would be worn by on duty security personnel.
Duty Uniform: Everyone would wear this during regular duty hours.
Dress Uniform
Physical Fitness Uniform

Other clothing items would include:
Away Jacket
Various types of head wear

Things you'd see on the uniforms:
Branch Insignia: like the medical cross, command star, etc seen on older trek.
Name tag
Decorations: you'd wear with dress uniforms.
Federation Flag
Unit Insignia: Symbol of the ship you're on.
Skill Badges
Rank Insignia