[RoundelM3]

The S65 swap has been done - very expensive and probably not worth it. Moving to an N54/N55 would be less expensive, but still costly and, as with the S65, will require a complete ECU swap plus the addition of an oil cooler and intercooler. Personally, if the wife's 128i M-Sport 6MT convertible's engine died, I'd probably replace it with another N52 - a lot simpler and no electronics/oil cooler/intercooler/exhaust plumbing swap-out/addition required. You could make some modifications like ZHP cams, 3-stage intake manifold, headers, software upgrade, etc. for a lot less than a turbo engine or an E9x M3 V8.