I prefer tear off, insulate, and then drywall. Bucksfan has a point that I have seen some old houses that don't use milled studs so they can be wavy which may mean shimming. Find a wall in a closet or out of the way place and tear it off to see what you got.
Plaster requires someone who knows how to fix cracks and skim coat it. They are hard and expensive to find. Learn plaster techniques and DIY. Just practice skim coating till you get it right or you have reduced the size of your room so you can't stand anymore.