The Focus RS engine is a good way to achieve a turbocharged Zetec using a lot of factory bits. The engines aren't particularly cheap though, and you'd have to buy an aftermarket ECU to run it.

If you go for the ST170 approach mentioned above it might work out cheaper. You'll still need an ECU, but the base engine will be cheaper.

All depends what you want from it, how much you're willing to spend and how easily you can source the parts. There is a thread on here somewhere outlining the ST170 turbo option so that would probably be worth a read if you can find it.