Why go for 8 valves when you can have 16????

Depends how much of the work you are willing to do yourself?

If you are the DIY type you can build a great 250 bhp zetec turbo for about £600 including management and turbo!

With the money you have in your budget 400bhp should be easily achieved.

HOWEVER, if you have no mechanical knowledge and don't wish to attemp building an engine i would triple those money figures.