I just saw this thread today so sorry for the late post.
They're supposed to be pretty solid other than a few engine issues. The cranks can't handle a lot of power and there have been some wiring issues I've heard about. Of course considering that you have another thread up about engine swaps then those problems won't be an issue for you. I know you were talking about putting a V8 in a miata and I highly recommend you really research what people have done to the frame and suspension. I talked to a guy with a supercharged 302 in one and he had said that it was nose heavy and wasn't too impressed with the change in handling after swapping in a v8. Now I don't know exactly what he had done to the suspension and if you can find something to improve that issue.
A v12 in a miata? That'd be crazy.
I saw a 5 series a bit ago go at a police auction for around 2k. It was a 530i with the N50 i think. For that price it wouldn't be too bad because you'd get the whole power train with it. A toyota 2jz or nissan rb would be pretty good as well. An RB20 wouldn't be too long.
An RB20 is a 2 litre inline 6 found in a skyline gts. I have never touched one but supposedly the crank is able to handle 400hp+. Nissan labels their engines pretty good. Here is some info for the engines:
SR,RB, etc is the block type
20,24, etc is displacement 20 = 2 litres, 24 = 2.4 litres
D means dual overhead cam
E means fuel injected
S means carbureted
T means turbocharged
TT means twin turbo.
If you look for a RB20 then get a DET version. Don't even look at another one. Of course if for some crazy chance you find a DET-R get that but I see no chance in hell of that. I still think you should look at a nissan SR20DET. It's a 2 litre inline 4 that can get crazy power and it's light so the handling properties of that miata shouldn't change much.