How to design and build intelligent controller for a power pack made of recycled lead-acid batteries?
In order to be cheap and work with different recycled batteries it would most likely have to use all the cells in parallel. Every new cell would increase the capacity a little bit.

The simplest way to connect them in parallel is to use some diodes or mosfets to make sure a shorted battery doesn't affect the pack. The circuit is something like this: 

Of course, the components should be chosen depending on the power requirements.

In some cases it might be more practical to have a board with a microcontroller which controls relays to disconnect each battery in turn every day and checks it's voltage.

Other than that, lead-acid batteries don't need an intelligent charger.