I had a really high spec XE, in a car I'd spent nearly ten years developing. I sold it all on to start again with the Honda Mega S2000 kit.
The kit has gone together like a dream so far, my only major issues were some delays in a small number of parts early on, which meant by the time I'd got them, it was the start of the summer and a mix of personal, club and work commitments meant I didn't have so much time free. (Oh and an easily resolved gearbox issue caused in transit - easily avoided if I'd collected it myself)
As far as ease of building goes though, the Mega car is at least as easy, if not easier than a Zetec or Duratec. Why easier? Because you've got an engine and box designed to bolt together and be used in a rear wheel drive car, unlike the Zetec and Duratec installs where you're converting front wheel drive engines to rear drive use, take a look at the posts on water rails and pluming, along with clutch combinations, to make the set up work.
Don't misunderstand me, both the Zetec and Duratec engines make fantastic cars, and all the extra work needed to use them in a rear drive set up has been done a thousand times before, so parts are available.
But the Honda just fits, and works, simple. Oh and to get the performance level of the Honda will take some expensive engine mods and engine rebuilds with either Ford option.
Don't forget, once you get away from the engine bay, the builds going to be virtually the same whatever engine choice you make anyway.