I'm no expert as I've not built mine yet but here's my outlook on it
I'm using a BMW V8 and from everything I've read and researched, engine wise you can choose whatever you can fit in the space and marry up to a gearbox and diff.
The problems arise if you want to deviate away from the sierra/MX5 donor for hubs/uprights etc as that needs modifying the suspension.
I'm sure I read some threads about using the 3 series Hubs but I'm not sure they ever got completed.
I'm sure someone with a bit more experience will add some more insight soon tho
