How should I approach body prep?

best bet is ask the guy thats doing it , everyone works differantly and the last thing you want to do is prep it the opposite of how he expects it

TBH, I'm not sure how much time you will save them. its easy to mess prep up. if its as simple as sanding the gloss off the existing paint then this will take the body shop like 10 minutes to do so will save you like 2p.

if you see bubbling you should cut it out and weld in new metal. Bubbling is rust coming through from the back of the panel.

Not knowing what you are working with makes it difficult to advise

The only body prep you should do before sending a car to the painters is to strip it. As in, take off all the chrome, the lights, the bumpers, the glass etc. That stuff is fiddly and annoying for a shop, so a lot of dodgy brothers out there just mask around them. It's also the part of the job that can really add up the hourly labour charge. Do it yourself, that way you lessen the risk of having the little bits grow legs. And if you want the engine bay sprayed, leave your engine at home when you drop the car off at the painters.

Example: A couple of years ago I worked in a panel shop. There was a HQ Kingswood that I had to get the lights and bars out of. Normally a few seconds job, a couple of screws and bolts and they fall out. But not this particular one. The bolts in the bars had seized. Ended up taking a day to strip the bars out of it. It was one of those cars that just fought the entire time.

As for sanding - unless you've had practice, don't. Partly because you're not going to save that much by doing it. Mostly because sanding isn't as simple as you think. It's not a brain-intensive job, but it's one that's really really easy to stuff up. And the last thing you want is to stuff it up so they have more to repair. If you want to learn how to sand, hang around a friendly panel shop and get them to teach you. For the primer, all paints are chemicals. Every brand makes them differently, and some combinations can cause problems. Let the painter choose every step.

The only time I've ever taken a car back to bare metal was for welding repairs. If the paint underneath was good and well adhered to the metal, it was simply sanded, primed and painted.