I did a end of year project on suspension kinematics at uni, but the more I learned the less I knew...
... this site has some awesome articles and is a good place to start, I wish I had this resource in 2004!
From a design perspective, the E90 stuff is pretty hard to beat. Multi-link is a massive step up from semi trailing arms, but the calculations required for things like spring rates are massively different - See link above.
Fortunately the E90 stuff (complete, including the front sub frame) has been used front and rear with an S85 in the past, although it all went into an E30 M3, so it "is" possible. See the pics attached for a possible solution - custom cradle for the rear using the same E90 pickup points, and then get that mated to the shell. Wide body will be the next step if you keep the sub frames the same width though.
If you're looking at the simplest solution at getting the power to the ground, I'd suggest a 4-link and tubbing the rear to the width of rubber you want to run - there's really nothing wrong with McPherson struts up front, and you can always build some custom coil overs with rates to your fancy. I've seen it in the rear end of an E30 with 650whp and it was an economical solution to the trailing arm problem.