Two things you've said here scare me. Lead paint is a concern if your home was built before 1973. Even before then, people were aware of the danger of lead paint and it's use was limited. There are cheap test kits that you can buy at a paint store that consist of a small paper tube with a brush on one end. Inside the tube is two glass containers containing chemicals that, when mixed together and sucreted into the brush, will turn red if it comes into contact with lead. This is not a diffinitive test, but it is good enough for this purpose. There are other things that will make it turn red, the good news is that if it doesn't turn red, there's no lead there. If there is lead, there still isn't much of a problem. People get into trouble with lead when they disturb it. You want to create as little dust as possible, prefferably none! Remove the loose areas of paint with "Peel Away" Paint remover and then treat with vinegar as before, then apply a good primer suitable for a masonry surface, then use three heavy coats of a "Lead Barrier Compound" This stuff isn't cheap! It'll cost you around $55.00 a gallon and a gallon will only cover about 150 SqFt.
What really concerns me, though, is the crumbling wall. With out being able to look at it, I can't really say for sure, but I suspect that what you need is a mason, not a paint job. It probably wouldn't cost anything to get a mason's opinion. Ask your friends if they know a reputable person and get an estimate to repair the crumbling and get an opinion as to why it's crumbling.