Knowing the age of your home, I would be very concerned about layers of lead paint underneath. I definitely wouldn't sand the walls without first checking for lead paint with a lead tester kit. You can buy them at home improvement stores. Unless the paint's chipping off, I wouldn't even sand them other than a light surface sanding for new paint. Yes, there are electric paint removers that will sand off the layers, but that's not going to be easy. It's going to produce a huge mess and the plaster will be damaged. If the existing paint's in decent condition, but not smooth, consider skim coating the surface instead of sanding through the layers. You could tear the old walls out and replace them with new drywall, but that will be a big project too.