I am planning an RTW but haven't decided yet which bike to use. Have the opportunity to buy a 1988 BMW R100GS. It has 95000 km and should be in good condition. Price 3500eur. Should I consider this bike or is it just to old?

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!

Thank you very much for your complete answer! I will try to figure out the history of the bike.

Hi, I bought a 1986 R80G/S with 45,000km on it and 10 previous owners. It successfully completed a 51,000km trip from the UK to Capetown 2 up over a period of 12 months. The bike only gave me 2 minor breakdowns. 1st the throtle gear stripped one tooth in Jordan and 2nd a bit of dirt in the carb in Namibia, both fixed in 1/2 hour by the roadside. A German guy on a 50,000km R100GS did a similar trip and his only problem was a burnt out rotor in Sudan. DHL had one to him in 3 days. I am now rebuilding my 96,000km bike and have found no wear in the top end of the engine nor in the gearbox. The only things I am replacing are the steering head bearings, wheel bearings, Gearbox bearings and springs( but only because I have it stripped down now)And fork springs. Regards Chris