I'd say, unless you go the MX-5 route altogether, or stay with the donor Pinto or CVH, Zetec is a good choice. Easy and cheap to get ~160 bhp out of it (bike carbs and a reasonable exhaust), and ~180-200 is not far off, too (fast road cams and bike throttles). Will mate with the Sierra gearboxes without much trouble. If you break it, the replacement will set you off ~100 quid or so.
So, definitely look at the Zetec, unless you have an aversion to four pots or Ford engineering.
More info here:
http://www.toyne.org.uk/zetec.html