When to replace Caliper Unit? In Addition to pads and rotors?
 

At what point do you need to replace the brake caliper unit? First off Nissan frontier, first truck, with only 22k I let it sit one winter (may have frozen the caliper). This is Concerning the Rear brakes; I started to feel the pad go. Then I felt erratic sound coming from the right rear. So I needed to get a closer look but in the meantime I may have waited a bit too long. I took both wheels off noticed I lost the brake pad and plate completely. I noticed I wore .020 off the caliper evenly (NON- Piston side). So I plan to replace both rotors and pads, can I get away with not replacing the caliper themselves (right rear only)? I never installed oneon a truck or car for that fact. Or am I setting myself up for more complications by ignoring it now and possibly having to do it later anyway?
PS If you all recommend I do it, Can only one side be done? Or suggest both? and How complicated is it? Seems like it gets replaced from the Banjo fitting on, (including full brake fluid replacement )