It's very unlikely to be a disc fault: aftermarket ones usually come out of the same factory as o/e, and it's easy to check for runout; just spin the wheel and eyeball the disc. It'll be obvious if its warped enough to cause overheated brakes.
You have a sticking caliper: it's rarely a problem with the caliper which causes this, it's a symptom of a faulty brake hose. This is pretty common- the hose breaks down internally, and acts as a one way valve for the brake fluid; consequently, as you release the brake the fluid can't get back up the pipe and the brake stays on.
It's easy to check, remove the flexible hose and blow through it both ways; the resistance should be the same but with a faulty one you find its harder to blow one way than the other.It's a simple test, and can save the cost of a new caliper. Check it first.