You're planning to run the Bosch 400's @12V. That gives you 2 realistic options for LiPo. 3S or 4S. (5S is 18.5V, right in the ballpark for "overvolting", but then the gearratio is a tad low). A 8000mah pack will probably see you trough a fight.

This means you need 1 Imax B6 AC , cost, less than £50 delivered at your door. And that one can charge the 4S 8000mah pack from "safely flat to full" in less than 2 hours.

Pricetag together, a tad more than 1 45Ah gelcel or AGM SLA without charger.
Difference in weight..... huge. 1 kg compared to 12 kg.