As BJH said, try to avoid rusty blades, although I have found those with just a light coating of surface rust will generally clean up OK with a little steel wool and WD40. The better made hand saws were taper ground. They are thicker at the teeth than at the back of the blade. If I were looking for a good handsaw to restore, that is one of the things I would look for.
For scrapers, I would try to avoid taper ground saws. If you do have to grind or sand the face to clean it up, it is as important to dress the face to a fine finish as it is the edge. A coarse finish on either will result in a sawtooth effect at the working edge, which will hamper the performance of the scraper.
Laurie