Congrats. Funny to think $55k + BAT fees is a steal, but I think you bought that car really well. I really liked the Chiarretto / Caramel color combo. Its not just rare, but very pretty imo as well. I frequent a smaller car message board where we were tracking that auction. Everyone there was predicting over $60k. We wondered if the green one listed at the same time competed for buyers with this one. Was the buyer OK with the price?
I had an '03 M5 for a few years. Absolutely great car, one of my favorites ever.
I sold it in 2013. My car was really solid. It seems like the s62 engine was more robust than the m62, so I didn't have as many of the normal v8 BMW issues as the 540 people did. Watch out for leaking rear view mirrors. The chromatic goo in them will do a number on your interior parts if it gets on anything.
Enjoy. If there is anything I can answer, I'm happy to. It's been a while, but the beast still holds a place in my heart. m5board is a great resource.