I reckon both the sierra and mazda have good and bad points but the platform of either is a good foundation if you want something a bit special.. Both of the above would make a decent daily driver but with the amount of engine choice out there who could not resist the urge to mix and match to the point of not needing the donor in the first place.

Bob

You will only need a cat if your engine is from 1995 on (August or September 1995 IIRC, but not 100% sure). That means all the 1.6 'short nose' crank lumps and early 1.8s would not need a cat. There is plenty of tuning knowledge and several options wrt ECU choice and set-up if you do modify the lump.

When I was searching for a donor, I found that the price difference on the open market between a Sierra and a MX-5/Eunos Roadster was negligible.

Lots of the unusable parts on a MX-5 can be sold for good money if they are in good enough condition, especially if you buy one with a hard top or if it has certain JDM parts fitted. The standard MX-5 alloys are also very light weight; you would have to spend a lot of money on getting lighter aftermarket alloys.