We all have different views and ideas it's a bespoke hand built sports car my opinion is mx5 is readily available I went for mx5 and made my own version of Phil's rear diff cover if you look at my pics but use the Sierra rear end frame so you could swap to a Sierra diff at a later date.
Why do you want to go duratec though?
A nice jap engine are pretty much indestructible if it's power you want strap a mini supercharger to it or do a turbo conversion later! (That's my route)
Whatever you decide there is lots of info between the guys on this forum that can help in your build!!