Questions: 2nd battery for my sweet, sweet minivan?

You'll need an isolator, which allows you to use the AGM battery and not draw down the van starting battery. Also something to secure the battery to the floor so it doesn't go flying around on rough roads or in an accident.

The rest of your list looks pretty good, except that I am not sure about those wells being anywhere deep enough. My AGM batteries are pretty tall.

The experts who will chime in soon will give you a LOT more advice, probably better than mine. Good luck with your project.