look up Allegro.pl then go to Automoto and search the cars (prices in Polish Złoty), it will give you an idea of what is selling for what, 3500pounds is a decent enough sum to buy a 1998 VW Golf for example
Definately buy a second hand car somewhere else! Anything second hand, not only cars, sells for totally unrealistic prices in Poland.
I regularily visit the flea market at Kolo and just cannot believe the prices being asked for junk.
I would do what Giles suggests or try to buy from an ex-pat who's leaving.