I can tell you what I know from shopping for my Xi sedan. Your list for the sport package looks right. Sport seats have adjustable bolsters so they grip you great and look pretty cool too. The suspension on a rwd is not only lower but also lighter. The xi does not get that tasty bit. The 18"s with summer performance tires are an extra option on top of the xi sport pkg, but it looks like it is standard on the rwd, which is very good for you because that nets you the aux oil cooler. Much debate over that one on this board, (search "oil cooler" and prepare for much reading) but I think a little extra protection is always worth it. If you are shelling out for the sport package, the cooler is one of the best parts. As for iPod, the car does come with a jack, but do yourself a favor and get the factory interface. It is worth every penny to be able to control it through the radio and not try to fiddle with the iPod itself while driving. Don't know if the interface is a DIY project, but I think it costs $400 for the part itself, so DIY would save you very little.
Hope this helps. Good luck with your purchase!
335xi E90, 6MT, Crimson Red, Black'ette, Sirius, 6FL iPod, Bluetooth