IMO, you're on the right track with your prep. Are you changing color? Look carefully at what's there now. Note runs and "bald" spots (like the seam under the firewall). Imagine it with a fresh coat of paint. If you're changing color, imagine a shiny coat of the new color. The tendency is to take it down to bare metal and try to shoot a perfect coat. The factory guys tended to whiz through it making sure things were covered but not to the standards of the outer body. They knew an engine would be in there that would be blowing road trash and its oil from its own leaks.
IMO, it does not take a lot of skill to paint a vacant engine compartment largely because of the runs and bald spots I've seen in them. That's why I painted the underhood and trunk interior on my Demon and had a pro shoot the body. If I can do it, surely you can.
Do make sure that every area you're going to spray is free of dirt and dust. The stuff looks like what it is just covered with paint. Try to use paint from the same mix as the body to retain consistent color. The paint stock used to make the underhood paint should be the same as used on the body. If this is not possible, get enough to do the entire underhood area to retain the same color. (When I say color, think shade and hue, not red and yellow.)
