When you are building an electric plane and using your TX along with its receiver, do you have to worry about the brand of ESC, speed controller, and motor? How do you decide what size motor to buy?

You can use any brand of esc, with your radio. You may need to reverse the throttle direction on your radio but not a problem. Motor sizing depends on alot of things such as weight of plane, prop size, etc. General rule of thumb is 75 watts per pound of aircraft weight for sport flying. Go to the electric forum and look for the beginners sticky lots of info for all your ?