Don't know a lot about Clarion, but here are a few helpful pointers....
First the answer to your question: "1DIN unit, no hideaway box" means it is a standard DIN sized unit (which is good for you because BMW is single DIN as well). As far as the no hide away box, most mobile video units have a seperate hideaway box that has inputs for other sources (CD Changer, DVD Player, etc.). This one obviously has all that built into the back of the unit.
Just keep in mind that unit appears to be a screen and an AM/FM tuner and that's it. You would need to add a CD changer if you want to play CD's and you would have to add a DVD player (or a PS2) if you wanted to watch movies. IF not, you could enjoy listening to the radio and stairing at a blue screen.