Before making a rash purchase and buying a common 152 I would advise you to give the thrill of open cockpit flying a chance and get a Tiger Moth. You could get one for about £35,000 which is more than 15-20 k but if you think about it, what kind of a plane would you be getting for that? My "guestimate" would be a possibly dangerous one which could easily end up costing you loads in maintenance and you might even have to get the engine overhauled or something