This is exactly what I did. I wanted to learn DC circuits and battery technology and I did that in spades.

Batteries are really simple, just need to respect them. If you are going to use recycled batteries, I suggest you buy a Opus c3400 tester. Run them through a complete charge/discharge cycle to identify capacity and bad cells. If they get hot, take them out and properly recycle them at Autozone. Otherwise, write down the capacity on the cell.

You can use repakr once you have 100 cells or so. I went with a 13s6p configuration for a 48volt system. I learned a ton from Micah Toll's DIY Lithium Battery book. Also, register at Endless-Sphere and start reading. Eventually the information will coalesce and you can start to buy bits and bobs. Take your time and don't rush buying right away... lots to learn.