Yes, American Head units are OK in UK, but if anything goes wrong the warranty will almost certainly be void. This has never put me off in the past though!
I also have an E46, and have components up front, and a sub in the back. I never bothered with the rear speakers. However, if you don't want a sub then the rear speakers are the best option.
What I would do is get another set of the MB Quarts and install the bass and crossover, but leave out the tweeters. This will leave you with a better sound stage at the front, and obviously the rear and front will match tonally.
Also, in the rear there is a sort of bucket thing that the speakers sit on. DO NOT REMOVE THIS. It also supportd the rear shelf! In the bottom of the bucket there is some carpet. Remove the carpet as it restricts air flow. This will help the bass.