I have an H8 (Champion H8-900CHAGM) in my 2001 3.0i.
I had really wanted to put as big as possible in there, specifically an H9 so I'd be able to swap for debugging purposes with an H9 that my Cayenne can use. After I concluded the H9 would not fit in my X5 (I got in there and measured, looked, thought about what I could do with some Dremel tool work on the plastic, etc.), I decided on H8s for both the X5 and the Cayenne.
So I would ignore anything about CA, CCA, etc. Get an AGM of the biggest size that will fit, and for my 2001 3.0i at least, that was an H8 (that is a spec on the dimensions of the battery).
BTW, the Champion brand is made by Johnson Control (they have a new name following sale of the company a couple of years ago) and is the PepBoys store brand.
Warranty and easy use of that warranty with local store availability (e.g., vs. buying from Amazon) for a test and free swap is a big factor - so PepBoys, Home Depot, Costco, etc. are all good choices there.