I used to wrench on Beemers at an independent shop. Came to love GSes over all of them. I'm no font of info as I've been out of the game for a long while now, but I'll tell you this: you probably want one, and you'll probably love it.
As soon as I have the scratch I'll be buying the best '04 I can find. Aught-four was the last of the oilheads, and in typical BMW fashion, they killed the model as soon as they had it all figured out. The dual plug heads ('04 only) are very nice to have, though not necessary by any means. The ABS and servo-assisted brakes are good but complex. I've honestly never felt the need for them, and I'd give serious consideration to removing it all and going to straight juice brakes for the sake of simplicity.
Final drives can assplode if the bike is overloaded and overheated - which is not uncommon with a bike that attracts guys that like to buy enough luggage to fill a train car, fill it up with everything they own then ride across the country at interstate speeds without stopping in the middle of July. Not exactly a cheap fix, either. But, refrain from that sort of activity and the rear end will never cause you a problem.
Hall (crank) sensors are hung in the front of the motor in what strikes me as a fairly vulnerable location, but the engineers didn't ask my opinion. Hall sensor failure is one of the few things that will stop an oilhead dead in its tracks. Easy fix, though.
Find one with a good maintenance record that doesn't look like it was abused or regularly overloaded, download the service schedules (5k minor is a glorified oil change, 10k major adds valve adjustment and trans/rear fluid change), buy some ball-end hex drivers, follow said service schedule and the GS will stay happy for a long while.
As for hexheads from '05 and on, well I'm wary. First of all, they use a CAN/BUS electrical system that strikes me as complex and frightening because I don't understand it. There's no drain plug for the rear end and they claim it's lifetime fluid, even though it's almost all the same parts and definitely the same fluid as the last model, which they told you to change every 10k miles. They're really nice bikes to ride, and I'm guessing the kinks are pretty thoroughly worked out by now, but I don't think they're as DIY friendly as the oilheads are.