I'm not sure what the differences are between the '02 and '03 Trek 5200 besides the paint color. The '04 has a few changes over the '03. The '04 5200 has the '03 Trek 5900 handlebar and stem... it's the 31.8 (larger bar). The '04 also has a better seatpost. That's it! You could buy a better Handlebar/Stem and seatpost for less that $250 and put them on a '02 Trek 5200 and still come out much cheaper than an '04 Trek 5200.
Sounds like a great deal on the '02 Trek 5200. If you plan on selling it sometime down the road... you may not get as much for it as you would a '04 model just becuase it's a '02 model.
I personally bought a 2003 Project One 5200 for $3100 tax included. I had many upgrades. Seat, Handlebar/Stem, Aero Wheels, and Custom Paint. I also got to pick the size of the crank arms, cassette, etc... My 2003 P1 Trek 5200 is much better than a stock 2004 Trek 5200 (the frame is the same).