Should add, just for clarity, my comments referred to the Zetec on OEM plenum etc that the OP mentioned, NOT the Mazda engine!
If, and it's a big if you're fairly confident that you'll not want to do huge amounts to the car, (or are happy with the idea of trading up cars rather than modifying), then for anyone wanting the minimum of tinker time and maximum driving time, I would say you need to keep the car and it's components as close as possible to how the original manufacturers intended them. Personally, in the current WF line up, I would say by far the best way to do that is with an MX5 SDV car. A standard MX5 engine, (or lightly tuned), as fitted in a Westfield, seems to be much less of a compromise than a boggo standard Zetec. (Not having a pop at Zetecs! )
It's only if you plan on steadily developing a car over time, or want to buy into an already developed car that a multi donor vehicle starts to make more sense.
The important thing, is that either route will be bags of fun!