What if the prior owner did it wrong? Copying what was there assumes what was there was "right". Getting chain length correct is super easy, Sheldon Brown site details the method.

As far as a freewheel, I assume you have a 5 speed. I would just take the wheel to your favorite LBS, have them remove the old one, and install a new one. There are dozens of different freewheel tools out there (lack of a standard). So unless you plan to work on this particular freewheel again, the tool will be a one time use (replacement freewheel will use a different style tool!!). I have a drawer full of freewheel tools, and still am not close to having them all.