Is there a reason for wanting the smallest amperage you can get by with?
A good battery is well worth it's price if it's got the juice to crank your engine every time. Scrimping on a battery to save a few bucks could leave you stranded or you could end up buying another one if a bargain battery dies on you.
Assuming you don't have a lot of electronics running with the engine off or a fridge, I would just buy two 27 series batteries. The weight difference between a 27 and a 24 is not enough to worry about.
