The second question is: To repair or restore?
Some knives may only have a broken spring or scale. These are repairs. In the course of making repairs, a full restoration may be in order, as the knife may have to be disassembled for the repair to be made. For example, a broken button spring repair requires almost total disassembly of the knife. It may be economically feasible to continue the process into a restoration, as labor costs are reduced. Most spring replacements do not require disassembly.
Any repairs should not be obvious. Most collectors would rather have a pitted blade than one with the lines slurred from inexperienced polishing.
Many times a knife only needs a good cleaning, tightening, and proper lubrication to function properly. Make sure you try these before paying good money to repair. Many years ago, at the Dallas Expo gun and knife show, a gentleman sold me a whole box of "junk" knives. Two hours later, with the help of a can of WD40 and some steel wool, I sold about half of them as working pieces. I paid for my trip, table, and hotel room with the money!
Use common sense and consult an expert if in doubt about repairs. Good photographs are a big help in evaluating a knife for repairs.