If its got a current MOT and the brakes feel ok, I would drive it round a bit more to find out all its faults...
ABS, stalling, Low MPG, 5.5k RPM, gearbox, seized rear calipers
Once you have a good idea of what's wrong then i would spend the £300 accordingly.
If its got bent jacking points then get the sill covers off asap, most Mk3 GTI's I come across now have rotten sills and brown crispy jacking points, this costs a lot to put right.
Rust is the biggest enemy of a Mk3, mechanical/electrical stuff can be bought for pennies and fitted easily but body work is always expensive and requires a pro.