Great instructions by the OP. Had low battery warnings and increased battery discharge warnings in past month as it's been getting colder in 2011 X5 with original battery.

Just got a perfect fit 92Ah Duracell AGM battery from my 35d from BatteriesPlus.com (10% off coupon code) for $200 and installed without a glitch. Make sure you get the same battery size and type (AGM vs flooded lead acid) as what you had before--mine was group 49 AGM. All you need are a couple of standard metric sockets, socket extensions, and a Phillips screwdriver to remove the old harnesses and brackets--took about 30 minutes taking my sweet time.

After I followed the instructions using BMW's INPA software for battery REGISTRATION (takes about 2 minutes going over the menus carefully as posted by OP), I was good to go. The BMW INPA software I got eBay for about $30 shipped with a OBDII KCAN-DCAN BMW USB cable (obdinnovations.com)--which you can use to code your car as well for other feature options (although a bit finicky to set up).