Not all old wiring is a problem, so don’t make that leap in judgment when looking at older homes; the best way to proceed is to have an electrician with experience dealing with vintage wiring to determine the state of the wiring in the home before you sign on the dotted line so that you know just what sort of upgrades you might need to do.
Well, it just so happens that you don't have to chew on a painted wall to get lead poisoning from the paint on it. It is true that one way that you can get lead poisoning is from ingesting paint chips that have come off the deteriorating surface of a painted surface but that is less likely to occur than from exposure to lead dust.