I would register the battery because if the old one was going, and not accepting a charge as a new one does, the new one can get overcharged and subsequently fail. When I bought my last battery the supplier asked if it was getting registered, to qualify the warranty. You can probably get away without doing it, until the time that you don't. No way to predict.
Your independent is inflating the price of the brake job. My local BMW dealer advertises service prices on line. For your 2008 X5 3.0, rear brakes including rotors and sensor, with labour, is $550. Fronts are $575. Two year parts and labour warranty. Taxes extra. They have an express service with 90 minutes turnaround. Call a local dealer and ask for a quote.
If you want to do it yourself, compare by pricing OE pads and rotors from the dealer. Then, decide if you want to use aftermarket parts and reduce the price further.
The computer estimate of 1700 miles left is not exact, but does suggest they are getting pretty thin. Sure, you can leave it a few weeks. But going past the lining gets expensive fast. Part of deciding if you are going to tackle it yourself could include jacking it up, putting it on jack stands, pulling the rear wheels, and seeing just how thin the pads are. You can see after pulling the wheels. I don't have the minimum rotor thickness handy, but that is what the 18.8 mm measured thickness relates to.
If you are going to do it yourself, realize that brakes are a safety system. Many claim they can slide a set of pads in, and be done in minutes. All that means is that they have replaced pads, not overhauled the brakes. Doing it properly means inspecting the rotors for minimum thickness, and possibly replacing them. It means cleaning everything, using the correct high temperature grease on the caliper ears, not lubing the guide pins, and so on. It also means doing it safely, ie not just on a jack. You may need to purchase some tools. Best is if you have a friend who has done it before that can do it with you.
To familiarize yourself with the procedure, this is the best link on the site for a DIY brake job. It is specific to the 2001-2006 E53, but is very similar to your vehicle (part numbers vary, for example). Read it and decide.
http://www.xoutpost.com/articles/x5/...e-job-diy.html
Good luck
Jeff