Hey can anybody help me with an issue I have with my front brakes and determining mileage?

1978 300D.

My front right brake pad sounds like it's hitting the rotor at every rotation, but only after I've been using the brakes a lot(downhill, freeway offramp, etc). Warped rotor? I had stuck pistons so I put in new calipers, changed brake fluid and new pads.

Any suggestions how to fix this? I'm trying to sell the car due to an overseas move.

Also, I'd like to figure out exactly how many miles are on the car. Someone installed an exchange engine assembly, hoses, starter, and mounts at 145,101 miles (now 221,083 miles). Can I assume this engine was new? Or did it have miles on it?