Plan on doing 2 myself next year, first will be a Zetec which is probably the cheapest and easiest as it goes on to the original gearbox, just need a sump, flywheel & clutch, water rail and inlet manifold all available and you can use the ford Ecu. Going st170 gets more power but adds to the cost and work involved. Both can have a turbo fitted later

Next is a 24v Cosworth v6, which requires a gearbox change, I have gone for a T5 with new bell housing and Cosworth 2wd propshaft which should be easier to fit, new flywheel and clutch also required and will probably need a new ecu, all in all an expensive way to go

The other option I would consider is Mazda, either MX5 complete or once the Mazda gearbox is installed it opens up more options