looking for Advice and Ideas

[PorkChop]

Personally the things I would be looking at would be the diff ratios you can get and how you'll do the reverse gear.

Plus, as you're doing a mixed engine/drivetrain car, I wouldn't worry about getting a specific donor; you'll struggle to find that many Sierras and the MX5 parts market is so flooded with cheap parts that you'll find it difficult to get any sort of decent money for unwanted parts.

Just to give some idea of MX5 parts cost at the moment...

a good condition 5 speed gearbox I recently saw had an asking price of £15. One of the MX5 breakers was asking £25 for their boxes. An open diff can be had for £25-50, brakes (especially early 1600) are dirt cheap, there's no call for the uprights apart from kit cars, driveshafts are £10-15 each...