duratec engine will cost alot more in conversion parts then the BP option.
If you plan to go turbo, find either a BPT motor out of a Laser TX3 or a import 323 GTX. The GTX option are available from quiet a few import wreckers.
Source the MX5 sump, inlet manifold and front mount thermostat housing. Easiest way to go about it. That's what i did.
Cheers
Zac