dchance and joe currier both provided very good advice. I'd echo the upgrade to a 24v system if you can afford it.
If you are going to stay with a 12v, 54lb system, I'd buy the largest capacity battery that will fit in your TM battery compartment. If you can fit a 30 series battery in that space, I'd go with a Trojan SCS225. If you can only fit a 27 series battery in that space, I'd install a Trojan SCS220. If you cannot fit a 27 series battery in that space... well... I'd look at figuring out a way to make it fit.
All the best,
Glenn