no offence mate, but if you are on such a budget you cant affort to buy the focus rs engine, then you are going to struggle to turbo the existing zetec you have. and if you do manage, it will probably be abit of a bodge up with used parts and no doubt wont work very well.
if you want a ford 4 pot turbo, have you looked into buying a FRST or ERST? maybe even a standard sapphire.