Have a good look through some of the postings below regarding the R100GS. I don't think there is a 'too old' cut off, it is more a question of how the bike has been maintained and run for that period. Certainly, parts of the original bike are used up and have been replaced or need to be. The KM on the bike and age brings into question it's maintainence history. I would suggest that such a bike would require a significant going-through to check for worn parts and prepare for a long trip. If the price is low enough to justify this, then it could be a good buy. It all depends on the history. Properly treated, much of the bike has lots of life left, but other parts are dead or seriously suspect (drive shaft, seals, swing arm and steering head bearings, etc).
On the other hand unless you are buying new or close to new (and even then!), any bike should have a thorough review and check over, and likely many parts upgraded or changed in preperation for a long trip. Again, it is really a question of cost vs condition/ history at this point and you would need to provide much more detail for anyone to help with your decision.
As to price, 3500 euros seems on the low side - but look closely at the cost of rebuild and parts before you assume it is a deal!