Wow, $900 bones for a BMW Battery? I've bought several over the years and even the big one was $260 last year. I haven't found any other brand that lasts as long, the OEM one it replaced was 11 years old!
If you own any newer car you want a bi-directional code reader, my Autel was around $350 but it is awesome for reading codes and resetting things like oil lights, maintance lights and registering batteries. I don't think you'll hurt the car without registering it but it won't save the voltage history which can be helpful when troubleshooting.