August in Europe is peak vacation time for the locals...and, often when model changes occur, so you may be on the edge of model production.
Engine tweaks for the 550 will not work with the 535, although there are a few things for the 535, there aren't as many as for the 550. Personally, I find the 535 plenty, but there are lots of people that want more.
Check out
www.tirerack.com to see what fits. While you may not choose one, they do give you the specs, and that can help determine what will fit from other manufacturers. Since all (?) recent BMWs are designed for RFT, they do not have a spare tire well, and any wheel you want should be capable of accepting a TPMS sensor, not all are.
I picked mine up in April, 2011, just before the airfare season change where the weather was cool, but the snow was gone. Spring there, is nice. Lots less competition at the tourist attractions, too.