i agree. if its standard height, stick to 15's. if you want to lower it do that before you fit the wheels. for your info i have a mk2 woth 60mm drop and i was running 6.5 x 17's for a bit. i thought it looked good but the ride was harsh. there are plenty of cheap tyres in 205/40/17 (have a look at mytyres.co.uk)
i also had some 6 x 15 tsw evo's on for a bit with 195/50/15 tyres. this was when the car had standard suspesion. the tyres were big enough to look good in the arches.
with a budget of £300 i'd get some lowering springs and a set of alloys off ebay. if you take your time and trawl through all the wheels, rather than searching "golf alloys" you will find a bargain sooner or later. my best find so far was a set of 19 inch mille miglia wheels and tyres off a freelander for £50 due to a terrible listing!