BMW 518 m10b18 to m20b25 or other?

Are the subframes / mounts different?
Subframe likely to be the same, mounts definitely will be M20 specific, use either E28 520i or 525e only

Will the 518 prop and diff be ok to mate with box?
I doubt the prop will be the right length to work with the M20 box. Easiest solution is to find out what exact type of getrag box was used for E28 520i manual cars, see if that is the same exact type as fitted to the E30 325i. Gear linkage will also likely be different. Apart from the engine you are better off getting the manual box, prop and gear linkage from a E28 520i manual.

Diff depends what ratio it is, as long as it's around 3.73 or 3.91 it should be fine. If not, use E28 520i MANUAL diff.

Will my steering box cause any problems?
I doubt it due to M20 being used in E28s. I can't remember if the M20 manifolds and exhaust downpipes used on E28s are E28 specific.

What ancillaries, wiring, pumps etc will I require.
All ancillaries and wiring from the E30 engine. As yours is carb and therefore is far less pressure than an injection engine needs you will need an injection spec pump setup. I would use whatever pump is fitted to M20 or M30 injection powered E28s.

Wiring from the E30 engine will need to be spliced into the engine loom going into the fusebox though this is easy once you find out what colours match to what.

And the big one...

Is it worth it?

It's probably not as easy as you think, M10 and M20 are very different engines but both were found in the E28. It would be a bolt in swap IF you found all the bits to do so, and the E30 setup will not provide all the bits to do so. If you were mega keen to do this I would buy just the engine and then buy an E28 520i manual engine mounts, radiator, box, prop, diff, gear linkage. The simplest thing to do would buy a 520i manual donor.

You'll need a exhaust system as the 518 exhaust probably isn't big enough, E30 325 won't work. E28 520i/525e exhaust should work or try using the E30 one and get a custom one made. The cooling system maybe an issue, I'd try using an E28 M20 rad from a 520i manual, otherwise custom fit one. I'd use an electric fan to save messing with viscous fans, Range Rover P38 twin electric fans are £20 delivered off ebay and are obscenely powerful. You will need to be able to fabricate, source all the bits and think on your feet.

If you had an E28 520i manual it would be an absolute walk in the park and done in a weekend EASY. I've done a E28 520i auto to M20B25 but kept the autobox, it was great and quicker than expected. I ended up boiling the M20B25 as the viscous fan wasn't right with the E34 radiator I tried to use, hence my suggestion on electric fans.

It's worth doing but is not that easy.

Thanks chaps for the replies, lots of useful info. Much appreciated.

Yes, that was going to be my next question! If it's going to be a pain sourcing parts and converting anyway I might as well go through a similar amount by doing an M50/52 swap. I imagine that a donor car would be easier/cheaper to source too? Is there an ideal car I should go for? Lots of manual e36 328i's pop up for cheap money I've noticed.

In all what parts would I require and will it just be the exhaust manifolds I would need custom made? Thanks again. Neil.

Either way this isn't going to happen next week.. More of a plan/parts list for when I've finally sold a few of my fleet and have my garage back.