First glider to buy 10-20k euro's

If you can accept a glider that has truly awful performance with wet wings then a Pik 20D might be a choice. It is quite amusing to watch - from another glider. And if you can accept that, and also a glider that doesn't have airbrakes, then buy a Pik 20B.

If you want to make a sensible choice then look at how many examples of older gliders were bought when they were new. The gliders that sold in higher numbers did so for good reasons - people who put their money into new gliders generally put a lot of thought into it - and that is still reflected in the current second hand market.