don't bother with an rs2000 engine, they're an I4 engine not a zetec.
don't need much really, just get an ST170 engine, change headgasket for a thick multilayer zetec one. fit some arp's on the bottom and get a turbo strapped to it.
uprate the injectors and pick an inlet, sort out some management and get it mapped.
that's essentially it for budget build.