I'd figure weight difference is minimal. A slushbox will add more weight than the doors, avoid if possible. The seats are different although interchangeable, but most of the body panels are unique. I'm told the hood & trunk are even different! I also heard that the 318ti shares sedan hood & front quarter panels.
I'd get it anyway...the e36 coupe styling isn't really much different, like the e46 or e90 is. I can't tell them apart from the front or rear!