The early Paralever drive shafts are, as Grant puts it, 'a service item' meaning that they have a working life span of +/-50000km. Lots go further, and some have quit sooner, but that is a good time to look at replacing them. Lots of good Airhead (air-cooled, 2 valve head BMW twin) on the Airheads web page http://www.airheads.org/index.php
and questions can be posted there as well.
The nice thing about these bikes is that generally, they are well looked after, being outside of the typical buy it and bash it owners group. Most GS owners are knowlegable about their bikes and maintenence etc. is done properly.
I am sure others will add more specifics, but as a start I would advise having a knowlegable shop look over any potential purchases. I mean a shop or individual with specific Airhead experience, not your average corner motorbike dealer.
You will find that many GS's have been 'upgraded' in some way, or accessorized:
Quality suspension upgrades are worth paying extra for - improved fork springs and aftermarket shocks are common and necessary improvements. the front brakes are not stellar, larger rotors, dual disk, and better calipers are nice. Watch out for any damage to the wheels. look for excessive run out, an easy measurement to do when viewing a bike. the wheels are difficult and expensive to rebuild. dual plugging is a nice option to have.
As with all bikes, the engine should start easy, run smooth, quiet (noisy valve gear?) and generally breath well. check carefully into any changes to the exhaust or carbs, as not all modifications are positive.
Generally great bikes that will last a long time, if maintained properly. Buy and enjoy. If I were you, I'd spend the extra money to purchase a low km PD model, as the extra fuel capacity is a must on long trips and an aftermarket tank will run you big $.