An Optima battery would be good as they are fully sealed, and they are a reasonable capacity (65AH) they are kind of heavy (40 pounds) but would work well for what you're doing.
Myself I use a 12v 35AH AGM style battery, it works better IMO since it isn't as heavy, and two of them cost the same as a single Optima.
Regardless of the capacity I'd spend a little more and buy something that isn't a wet cell, be it a dry cell like the Optima, or a AGM like the link above. They are easier to deal with, less mess, and will last longer.
You can then get an automatic charger like this one, which is inexpensive and just plug it in overnight and you're set.
I've actually got the charger above, it works really well. If you want one I've got an extra NIP.