Krylon 1613 is the best for the underhood area, looks almost exactly like the factory finish, cheap, easy to find, and you can touch it up later without it being a major project. I used Rustoleum semi gloss on the rear axle housing and the other bolt on pieces under the car and under the hood.
POR15 would make a great protective base coat, you must use their special paint before you can top coat it with another product. Preparation is the key to having POR15 stay in place and provide the protection you are looking for. Read and follow the directions carefully, use their special cleaner and their surface prep spray.