As an in field electrician, I also echo the advice to buy a pre-built pack. Batteries can be little mini grenades, especially when you overcharge them or overheat them. Go ahead, Google "battery explosion" and have a feast of firey goodness.

Bigger battery, means bigger fire. And at 300+ cells... thats a lot of battery.

This most definitely not in the realm of "an internet search, a soldering iron and some gumption". The whole install sounds closer to "get at least three quotes from a local electrician or solar installer" territory.