Im not sure you can combine those 2? Not all batteries are created equal and the "cheap" ones usually have a manufacturing defect. You really need to buy 10% more than needed and test them for capacity, then rack and stack em.
Once you got the voltage and amps you need, then depending on the chemistry you need a bms and charger.
I see a lot of lifepo deals on ebay, but I think I rather stick with a prius pack as they dont need a bms, just a grid charge to balance them out.
