Please help with Body Prep / Engine Bay

If it is factory paint, or if it is good quality paint, there's no need to take it down to metal since there is no rust. Otherwise you're spending a whole lot more time getting the paint off, correcting sand scratches, and spending $$ for a larger quantity of primer.
If it is crappy paint, you probably should play it safe and take it down to the metal if you want a nice job and plan on keeping the car for a while. Do one panel at a time.
If you want to do minor repairs, sand the repair area down to metal, a light spread of filler, and block it down even with the surrounding surface.