Much of the answer probably depends on the level of performance you want out of it, and how deep your pockets are. If you're after a useful increase in power and torque but don't want to go mad and spend loads of money, then a 2.0l Zetec engine will run very happily using a CVH carb, and will give around 130-135bhp.
There's no need to be wary of ECUs, millions of cars use them and do millions of miles with no issues. The great thing with them is that if you want to know exactly what's going on in your engine, just plug in your laptop and have a look!
My 4/4 is running a 1.8 Z18XER engine (used in Zafiras and Astras). It has an aftermarket MBE ECU programmed by John Ravenscroft (TVR) and gives around 170bhp.
