It's not nearly as hard to smooth up a plaster wall as you're probably thinking. Just get yourself a bright light and hold it close to the wall, but a few feet from where you're working. The sharp angle of the lighting will exagerate the roughness of your wall. You use that critical lighting to expose the bumps and depressions in the wall. If the bumps are small (like the size of a silver dollar or smaller, you can scrape them off with a paint scraper or sand them down. Depressions of similar size can be filled with joint compound.
Remember, the wall does not need to be perfectly flat. No plaster wall is, and most drywall walls aren't either. They just need to be smooth. As long as the wall is smooth, your brain will presume it's flat for lack of evidence to the contrary. And, if you use critical lighting to locate the high and low spots, you can make the wall smooth relatively easily.
If there was wallpaper on the walls, I would scrub the old glue off with wet green Scotchbrite pads before priming and painting.
You gotta remember that 70 or 80 years ago, the people who worked as plasterers were highly skilled, and would have made those walls smooth enough for painting. Unless they were damaged removing the wallpaper, they should still be smooth enough for painting. Gouges in the plaster are not a problem because they can be easily and quickly filled with joint compound.