1980 Capri could have 1 of 2 4cyl engines (1 being turbo charged), it could also have a straight 6 in it, it's also possible that it has the 255 V8 in it (which is a SLUG!). Being low original miles, I would recover the seats / new carpet / new dash pad (not just a "cover) - you can find these pretty reasonable from Latemodel Restoration to name just one company. I wouldn't get into changing the color as it can get VERY expensive! As for updating the brakes, one of the easiest / cheapest ways to go is to get the spindles from an 87 - 93 Mustang (bolt on swap) & then get the new rotors /calipers & pads to go with that set up (they are over an inch larger, yet don't require larger wheels & MUCH better than the 10" rotors these cars came with new). As for the motor; if it's either 4cyl you could rebuild or swap, with the straight 6 you're kinda stuck rebuilding. Doing a swap (4cyl or V8) would require swaping the front K member ( they are unique to the L6), as for the 255, I would suggest swapping - these were just about useless when new & there's little to no aftermarket parts available for them.
As your other replier stated, swapping can get costly, but you can determine how far you want to go in advance. Conceivably you could put a V8 into a car with the 4cyl suspension & then as things get worn, do the upgrades to the V8 parts (alot of the time there's no difference), I would do the brake upgrade to the later model spindes/rotors no matter what! In reguards to what your other replier said reguarding his son's firebird swap from 4cyl to V8, this would be the same as L6 to either other motor in your case. The 4cyl F body GM cars were only made for a couple of years as they were more or less "bastard" children & VERY costly to produce. Same thing can be said about the L6 Capri's & Mustang's, which is why they too were only built that way for a couple of years.