Be wary of 326M and 296M wheels if used. Both are notorious for cracking (especially rears, as they are wider).
If you see any that have cracked and been repaired (as it's perfectly legal to weld them) stay away IMO. They will crack again.
M3 and M4 wheels, 437Ms, are good as they are forged but are wider than E89 wheels and bloody expensive used.