welcome.
Performance wise GTI's have around 143bhp standard, 0-60 is about 7.5-8 seconds if i remember correctly and top speed is around 130. Add an exhaust and air filter you'll be looking at around 150bhp.
Fuel economy, i think the average is around 250 a tank, depending how you drive. It might not sound much but N15's dont have a massive tank, I filled mine with optimax the other day from empty to full and it was about 38 quid.
They're fun to drive and you wont regret getting one. What sort of age are you looking at? Personally (not being biased) id go for a 98 on model, exactly the same engine but a few cosmetic differences.