Re: AGM batteries
AGMs need to be secured. Doesn't hurt to put them in a box, either.
A group 27 start battery would work. I use a group 31 start battery for my 4-108 just for a bit of extra capacity.
AGMs and flooded batteries have almost identical charging profiles, so you could use a flooded start battery.
Not sure how your ACR is wired. The ideal way these days is to lead ALL charging sources to the house battery bank, then use either a voltage-follower device (e.g., EchoCharge or DuoCharge) or an ACR to maintain the start battery.
BTW, start batteries require very little charging. Starting your 4-108 typically will use 150-175A while cranking, but only for a very few seconds. Typical capacity draw for a diesel start is less than 0.5 amp-hours!! This is replaced in just a couple of minutes.