Have you bought anything yet fella?
If not, why don't you spoil yourself with 05/06 plate Gti, 60k miles. Should be around £7k.
You could then spend the remaining £3k of your budget on a whole host of mods, such as I did. The end result, an extra seventy bhp give or take a couple of ponies.
You seem keen on bringing down the 0-60 time and increasing top end and I can tell you it is possible with investment of around £3k to bring the 0-60 time down to around 5.5 seconds and raise your top end to 160 mph+
130M's, Cupra R's, R32's, they'll all be trailing in your wake mate and it's unquestionably proper good fun!