[M3-FAST]

I would not change a rear bumper without changing the front tbh - I think it would look odd?

You could poss add the small front lips to each side of the SE front bumper to make it look more aggressive, that way you would retain all your front fogs, etc..

There are normally quite a few M sport bumpers on ebay - you might get one in the correct colour?

You have gotta be careful that whatever you do is tasteful as you will get hit when you come to re-sell or part-ex. People may wonder why just the rear bumper does not match? You either need the whole look or none of it IMO.

Can't answer your questions about the Alpina stuff, sorry.