Some shops don't like someone else prepping a car before hand, I know I don't like it. A customer says its ready for paint except for these spots only to find out the car is a wobbly mess. You might be farther ahead to let the shop do it all.
Would it be best to just take it down to bare metal then let them go from there? I want to find a body guy who will let me help to save on cost also, I want to help and learn.