AGM batteries have been around for a while (before e46's). I would get an AGM, if the previous owner (my brother) hadn't just replaced the battery, I would have an AGM. I put it in every vehicle I own, including my motorcycle.
This is where I put mine too (under the hood), but I have my extender going down the side next to the washer fluid bottle. I open the hood an connect the cable and lay the hood in the open but down position. My car is in the garage, but if it was outside, if the hood was lifted, the alarm would go off.I wired my Battery Tender in the drug bin using the jump posts. Open hood, connect the 2 pin connector and route wires through the back of the hood seal next to front window and close hood.