I was putting in my track pads today in preparation for a track day on Tuesday, and the upper bolt on the right rear caliper was very difficult to take out. When I got it out, the last couple of millimeters were stripped, and presumably the nut in the boot is also stripped. The bolt will go back in almost all the way, and can be forced in tight (but not all the way in), so I'm not concerned with the brakes coming apart, but obviously this needs to be fixed.

Is it reasonably possible to replace just the nut, boot and bolt, or will I need to repalce the outer part of the caliper?

 

It sounds like the bolt that goes through the calpier mounting tab and into the slide-pin on the calpier bracket. The pin is surrounded by an accordion-like rubber boot. If that's the case, the pin and bolt are less than $5 total. A caliper rebuilt kt (~$20) will include both new pins, grease, boots, and piston seals -- but not the bolt.

 

The same thing happened to me during a pad swap. I replaced the pin and bolt and made sure to use high temp anti-sieze on all of the caliper fasteners.