Leave the VAG 1.8T's they're expensive to tune and don't really ever make good numbers, plus rods are like chocolate.
Loads and loads of options. Typically something Jap or Ford will be cheap and easy to tune but you will pay good money for a good turbo Ford these days. Some ideas based on a £5k budget:
S14a 200SX - Endless possibilities and parts avaliable, bullet proof engine, massive support and knowledge base. £3k will get you a tidy one so loads of change for
mods, easy 300hp.
Evo 5/6 - As above apart from you will need all £5k.
RX7 - Risky choice and need much pampering but look and sound amazing
imo.
Loads of Honda stuff but good power costs good money. Some very good cars out of the box (EK9, EP3, DC2/5, S2000 etc) but as a result they don't make for much of a project without making big changes.