Used parts present a compromise, in that their service life is inherently limited from time you get them, but a solid used part, properly prepared, installed, and maintained, should not be anything to worry about. Remember, these engines are known to put excess of 400hp to the ground on stock internals. They'll probably get you home on two cylinders and one forward gear if push comes to shove. It all depends on the condition and location of the used part(s).
What used parts are you considering?