Blast the whole thing! It's not an engine...the bore is not the sealing surface. It shouldn't even ever touch the piston. I have a caliper rebuilding business.....We do mainly air cooled porsche and vintage british. All of our iron castings go through an automatic shot blaster, we blast everything. It's the way it's been done since the beginning. Aluminum calipers with anodized bores...absolutely protect the bores and threaded holes at the very least to protect the anodizing while blasting...or simply forgo the blasting and do minor sanding before refinishing (if painted).
The piston in your picture looks like it was either not even wiped down and it has fluid and hair and crud stuck to it, or it's garbage. Piston should look relatively flawless, of you'll have problems.
The failure mode of just about every cast iron caliper that's come through the shop has been rust between in the bore between the boot and the seal. About the only reason I wouldn't blast the bore is if there WAS NOT rust in the bore between the boot and the seal, and there was still factory plating in place on that surface....then I'd lightly clean it with some mild scotchbrite such as not to go through the plating, and blast everything else. Most OE calipers these days don't have any plating in the bores though, so make sure you use a proper assembly lube that contains rust inhibitors (don't use brake fluid), and then get one of those little curved tip syringes they have to feed medicine to cats and fill it with silicone dielectric grease and squirt the bellows of the boots full of grease before you put it back together....if that "dry" area between the seal and boot is full of grease, it can't get full of water or dirt or anything else and caliper life will be greatly increased.
Clean the grease outta the slide pin holes in the bracket before you blast it....