will M3 springs fit right onto the 318 suspension?
Yea, might be stiffer. Not sure if it would give you much of a drop if that's what you were looking for.
As for H&R Kit, I would stay away from that. My friend has that on his 325 (although the 17" might play a factor into this) and it scrapes hard he has to SLOOOOWWW down way bad just so he doesn't scrape over bumps int he road and whatnot. Imagine that with the m42 oilpan =/