Hi Poppy,
If you can keep them with a top-up charge then shop around and when you see what you want buy them. They will last for years on top-up but if you discharge them flat, they will never be the same again. If you don't have them to hand, it will slow down your work as and when you need them. Serious faff if you have to borrow one from your other vehicle. Having them to hand will also help design where to fit them. Weight, room for cable connections and cable runs etc.
Richard