The running gear differences between the MK1 (NA) and MK2 (NB) are very few.
The mk2 steering arms are 7.1mm higher than the mk1
Suspension geometry is different but comes from the different subframes and wishbones.
The mk2 rear uprights are slightly different and will give a 5mm per side wider track. The drive shafts may be slightly longer, but they fit the mk1 so it's not a problem.
Engines, gearboxes, propshafts etc are also interchangable between 1.8 models.
The mk1 1.6 has lighter weight prop and smaller diff. Also smaller brake discs. Calipers are the same but different carriers.
MK2 gearbox is stronger. The 6 speed gearshift is not as good as the 5spd. The 6spd is stronger though. Both 5 and 6 speed gearboxes/diffs are known to be good for 300bhp+ in a 5 so even more when the Haynes is half the weight.
Exhaust manifolds are different from mk1-mk2 but only in length for the connection to the exhaust.
The ports are different on the head between 1.6 and 1.8 so manifolds aren't interchangable.
Diffs came in many flavours.
Most are open, but there are a lot of lsd's either torsen a, b or the viscous ones. Also the plate type that wear out by 100,000 miles so become open.
So in summary
I would go for a mk1 or MK2 donor with the 1.8 engine.
Use mk1 front uprights
5 speed gearbox
Torsen B diff from a mk2
If I didn't already have my donor I would be looking for a damaged 1999 1.8is sport (what I already have) as it has the diff and lots of good bits to sell like leather and hardtop.
Or look for an imported Miata 1.8 mk1 that is an early import (rusty as hell) as many of these were ltd editions with a lsd.
Phew. Sorry it's a bit of a ramble but I wrote it as I thought about it.
This link may help identify uk models.
http://ox.mx5oc.co.uk/guide_mx5.htm
I will give it some thought and maybe knock up a better donor buying guide.