Are you building the tank circuit to survive crank? Im assuming no since you have an M2. It seems here that you are mostly just trying to put an auxiliary battery in the back isolated from the starting battery. You can just go out and buy a battery isolator for that. Might want to consider a diode based isolator for reliability.
As for the battery, you could look into the optima blue or yellow top. The good thing about those batteries compared to standard marine deep cycles is that they can be charged at higher currents, so they should charge up faster. You dont want to take energy from the battery faster than you can put it in, so a faster charge rate is good for this application. Just something to consider.