Found fix to squealing noise... I hope.

A couple of tips to help with bleeding:

1) Minimize the amount of time the hydraulics are open. Don't disconnect the hose and leave it while you do other stuff. Get the replacement caliper, new copper washers etc ready and at hand. Then, unbolt the old caliper and immediately connect the new one. This helps prevent fluid loss from the system.

2) Pre-fill the new caliper as much as possible with fluid. Not unlike how you might fill a new oil filter with fresh oil before installing it, filling the caliper with fluid reduces how much air you need to pump out.

3) If the calipers are used carefully inspect them. Are the bleeders free? Do the pistons move reasonably easily? Any signs of leaking from the seal? Without disassembling them how do you know the condition of the bores and the fluid remaining in them? Consider a rebuild kit for them so you can inspect the bores and pistons, seals, clean them etc.

Everything depends on budget etc but I seriously doubt the wisdom of buying used braking components. Used calipers are probably okay with a rebuild but rotors and pads?