where you are, I don't think you hafta worry about <i>cold</i> cranking amps

but get the best you can afford - up north where it counts, 550 is min, 650 better, more's good too - when it's cold batteries lose some oomph, and motors are tighter to crank as well, but w/ the 6 volt starter on 12v, you'll have no problem starting - the alternator is what's important w/ the AC - make sure you've got a heavy duty one
Bill