hello mate, you shouldn't be worried at all, to be honest your not gonna have an issues with the engine or gearbox provided it's been looked after, there built like a german tank!
Just make sure the cambelt and water pump have been changed at the correct times,cos i know a lot of cars i was looking at around the 100k mark didn't have there cambelt or water pump changed but not to much of any issue as you can get it done by an audi speacialist or a garage you trust for much cheaper than audi would charge you!
i have a '98 a3 1.8t covered 95k when i bought it it had full service history,air con,bose system,leather,parking sensors,full audi history and i paid 2700 for it and it's mint and i have no issues or running problems, it's a solid car!You just have to keep looking and be patient to find the right one,have a look at a few before you commit,it took me 3 months and a load of canvassing before buying the car i have now, but it was well worth the wait.
plus if you look at cars around the 2500-3000 mark you can always get them down on there price if you have cash in your back pocket :icon_thumright:
i've had my car for little over 5 months now and haven't had any issues at all just pure motoring heaven and some modding.
Go for it mate!!