Hey mate,
The one I have with the factory ratio (still in the box) is out of an E24 6er or E28 5er (can't remember off the top of my head). The housing is the same as the E30 3er, but not the back cover. You will need an E30 cover. In addition there has been much talk about the axle fit. The best way is to take the Inner CV from an E30 axle and put it on the 318ti axle shafts. I'm looking for the link...
Read this...
http://www.emotors.ca/Articles/7.aspx You don't need the M-Coupe cover though.
Cossieturbo