Search for Max BMW online parts. That'll get you started in your parts search and give you the prices.
That rear drive stain is probably the rear pinion shaft seal in the final drive leaking. Common, but takes a special castellated socket to get it apart to jerk the seal out. I took mine to the dealer once I got it off the bike and had them look it over for wear anf if they thought it needed anything else while off. I think I spent $185 or so for the job. Fwiw, those pivot hex bolts on the shaft cover are tight (Locktight) and you may have to use a torch to heat them up to soften the bond along with a long snipe on a breaker bar.
The clutch slippage may be seepage from the tranny seal that gets pumped towards the clutch itself and not necessarily from the rear crankshaft seal. You'll know once you get it broken down. Many had it leak out the 'rear' tranny seal and flow up the middle of the shaft that holds the control rod onto the clutch. I'd replace all the seals in the tranny as they are probably dried out and shot. Heat from the catalytic hardens them up pretty quick.
I'd also bug your dealer for any updates to the parts on that old a model and get them installed too. They make minor changes that you may never hear about unless you tel them what you are doing. That control rod in mine changed design and I had to get some additions to make it work on my RT. Trust me, you don't want to pull it apart a second time. Mine took me about two weeks awaiting parts and sundry stuff to get it back together. You'll always find something else that could or should be changed once in there.
Good luck, and photograph the hell out of what you need to remove to refer back on later.