Need Help/Confirmation for Battery and Brake Change

[Bendingo]

Hi, everyone.
Nice meeting you all.

I'm a beginner in car mechanic and repair, but willing and capable to learn how to do basic DIY, and I would like to ask few questions to you experts.

I have an '08 x5 3.0si. It requires battery change and possibly brake change.

I recently went to a highly recommended indy shop in NNJ, and the quote just surprised me too much that I did my own research in this forum and other forums regarding the battery and brake change.

The indy shop quoted $326 for battery and $200 for labor.
He also quoted $160 for pads, $360 for rotors, $30 for sensors, and $260 for labor.
At that rate, I might as well go to the dealer and be done with it.

Anyways, although I'm a beginner in car mechanic and repair, I do not like to get ripped off. So, I decided to do some research to see if I can either DIY and bring the price down a little.

Now here are some questions.

1) I checked the battery, and it's a S: 61 21 7 551 331, 90 Ah, 900 CCA. I did more research online, and it's an AMG battery. I checked the Duralast-H8AGM, frequently recommended, and it has 92 Ah, 850 CCA. I thought the Ah and CCA should match exactly, or else, I'll need a "coding." Or is the difference negligible enough?

2) Is the registration absolutely necessary? I checked this youtube video, and the BMW mechanic said "If you have an E70 X5 that would be the reason for registering the battery. But I have not had a customer have a problem that did not have this registration performed." Considering it's only $50 to purchase the INPA cable, I'll probably do the registration to be safe than sorry, but I was just curious of your experiences.

3) For the rear brake pads, the system says I have 1700 miles more to go. The indy shop told me rotors measure 18.8mm and the brake pad is low. I do not know what this measurement means. Does it mean I need to replace them immediately? I do not want to prematurely change if it can last few more months since I do not drive much (less than 5000 miles/yr).

Anyways, I'd appreciate any advice or confirmation.
Thank you.