Though you don't give enough info about your truck (is it a crew cab, long bed, 4x4...??), it appears you're going to be just below your trucks tow and cargo ratings, even if 5er is maxed out. The cargo limit is the only fuzzy part. You're definitely safe in the tow rating category.
Yes, watch tranny temps and get an auxiliary cooler if you don't already have one.
I say get the 5er and rock n roll. Spend a few minutes on any PSD forum and you'll find that 7.3L owners are consistently exceeding their truck's factory ratings without any problems.
