Hi, Guys.
Here's an update. Sorry for the delay, but I managed to finalize everything last week.

I got
2x Zimmermann REAR Rotors
1x Textar REAR Brake pads
1x OEM Sensor
from AutohausAZ for around $250

Got
1x AutoParts H8 AGM battery
Advance Auto Parts for around $150.

I found out one of my father's new friend is a retired mechanic.
He did the job for $60 total. I guess my dad owes him a few dinners. LOL

He also said the registration is not necessary. In his experience and in his brother's experience, who owns his mechanic shop as well, there was never a problem without the battery registration.

But I just got the INBA cable, and did it anyways since it wasn't a big deal.

I was planning to flush the brake fluid with the BMW Brake fluid since that was actually cheaper than Pentosin or other brands. But after he checked the brake fluid, he told me it's unnecessary. Since I read everywhere online that biennial brake flush is highly recommended, I also asked a cousin who lives in Korea who used to be a car mechanic and now works for Lexus, and he told me for our mileage, that's not necessary. The 2 year mark is simply a guideline. In real life, it's better to check the brake fluid and your mileage. My x5's mileage is really low at around 30k for 6 years and probably less than 10k since the last brake flush. So I decided to wait a little longer.

Anyways, thanks everyone for your advice. I just wanted to post an update.
Thanks!