Overall there is nothing wrong with single stage paint(providing it is quality paint were talking about). It's typically easy too apply, cheaper, & holds out well. In some aspects I prefer the look for a older car- I sprayed my Demon in Endura single stage as I felt it looked more "correct" not the super glossy forever wet look that some choose to have(not to say that single stage can't look like that with a little effort). That being said there is one aspect to consider in a engine bay and that is the effect that fluids may have on it if spilled/ spashed etc and not cleaned all the time. Depending on the color the paint may stain & discolor whereas if it is cleared it does offer some protection against possible color change. If you are for sure going black you don't have much to worry about