450 should be fine. The battery booster pack is better then jumper cables because if no one else is around what are you going to hook the other end of the cables to?
I have a JNC12/24 booster in my work truck and it will sit for weeks at a time and still start a bus if they aren't completely dead.Yep, that's what I was thinking.
Any idea of how often you have to refresh the thing to keep it charged up?