If you have one of the single DIN beamer radios consider yourself lucky. If you can dig up a proper harness and get creative with some of the mounting bracketry behind the dash it will go right in. If you have one of BMW's goofy radios that is slightly taller than a DIN slot and like 1.5 times as wide, find an OEM or a good custom shop. Oh yeah, one more thing to check is the impedance of your speakers, some OEM beamer systems use really wierd super high or super low impedances and they won't work with an aftermarket deck so you'd have to change out all the speakers in the car too. Assuming you didn't get one of the wierd systems any aftermarket radio out there should be able to be dropped in in less than a half hour if you know what you are doing, less than 2 hours if you putz along.
<-- Used to work car audio