hi srl,
first off, did ur car come with the premium harmon kardon system (ie speaker system)?
i know i'm going to get flamed for saying this- because there are a lot of audio enthusiasts (cough: snobs)- on this site that hate the bmw premium harmon kardon system like it was their leprosy-inflicted mother-in-law.
my car came with the premium bmw head-unit + 6-cd changer + harmon kardon system. and, as both a musician and music enthusiast, -and- after almost 9 years of usage, i still say that i honestly like the audio quality it produces. it may not be the best bang for your buck and there are plenty of better things out there. but, it's not anywhere close to being as bad as the haters here are going to tell you it is.
the bmw hk system has many merits. first and foremost, it sounds really good (good quality). second, no one's going to break into your car to steal it (security). thirdly, it matches the aesthetics of the bmw dash perfectly (good looks).
now, for your situation- since you already lost the oem dash mount, u may want to consider an aftermarket head unit, since the aesthetics of your dash is pretty much altered already anyways.
www.crutchfield.com is a decent reference site for car audio.
www.bavauto.com also has some decent oem bmw audio components- such as 6-disc changer. (not sure if they sell head units.) if u have the hk system and want to put back an oem head-unit + the oem 6-disc, you won't be disappointed. if u don't have the hk speakers, then u will want to upgrade your speakers in addition to the head unit and 6-disc.
hope this helps.