Mk4 you'd need the 1.8T engine for any sort of excitement, as the 2l 8v is underpowered for such a heavy car. Don't know what Mk4 prices are, but I doubt £1500 will get you a well cared for and serviced 1.8T.
Where as £1000ish will get you a pretty mint Mk3 GTI 16v which has a great engine, is cheap to run, easy to work on and cheap to repair, so you'd have money to spare. £1500 will probably get you a VR6 highline in very good nick to be honest, but running cost will be a lot compared to the GTI.