Rebuilding old, frozen calipers is a dirty and somewhat difficult task, especially the first time you do it. You will need to do both sides otherwise the brakes may pull to one side. The pistons may be pitted from corrosion as is often the case with a frozen caliper. Sometimes you can polish a pitted piston enough to get a good seal with the new rubber. Frozen pistons do not readily come out. If you use compressed air they come out like a cannon ball. Don't have your fingers down in there.
Maybe check with our vendors/shops and see if they have rebuilt calipers on an exchange basis or if they will rebuild yours.