look into braille batteries. not cheap, but I have yet to hear anyone complain about them. I should have bought a braille, but I bought a 51r instead. I regret that choice. Do you want a mildly used 51R?
Look for some good cold cranking amps for a given size battery. The more amps the better it will turn over and the better chance you won't drain it trying to start it.
Any battery is going to lose its charge over time. A smaller one will empty itself faster. Its going to be hard to get around this--battery tender could help you out.