I am in the middle of building my Turbo charged ST170 Engine, my spec should see 450bhp+ it is a very expensive thing to do and im building it myself and doing the mapping. If you want to do it properly you will need at least 4K for the parts alone depending how you spec it.
Pistons and rods £700-£1000
Clutch £500-£1000
Turbo £900
intercooling and pipework etc £600
Afrtermarket ECU and mapping £1500+
Inlet Manifold £200-£600
Exhaust Manifold £300-£600
The ST170 bottom end is only ok for 320ish bhp so if your aiming higher then the pistons and rods need changing. The only difference between a standard zetec and ST170 bottom end is the pistons and rods so you may as well pick up a cheap 2.0L bottom end and build your engine from this and then fit the ST170 head to it. I managed to source a focus T230 engine which already had the high pressure oil supply tapped into the block for the turbo and the drain back into the sump. So thats another job that does not need doing.