I had an E36 M3. As far as I can remember, the rear subframe was much beefier and reinforced over a stock 3-series. I had an x-brace installed as an option. That's was pretty much it. Obviously you know about the upgraded suspension components on the M3.

Yes it is possible to convert a generic 3 series into an M3 given the means and materials. IMO, sorting out the wiring and installing the M3 flares would take the longest and require the assistance of a pro.

Short answer is there is little difference. Even the E36 M3 subframe can be retrofitted.