I did all the prep on mine, sanding, primer, taking off everything that could come off (took the drivers mirror and taillights when I dropped off the car) and did the trunk, door jambs and underside of the hood myself. Plus, talked with them about what exactly they'd need. I gave them one of the best prepped cars they had seen.
They had done classics before, and I had asked around about the different Maaco's in the area to find one that was competent and cared. It came out looking pretty good, until I got hit two months later.
So, prep and prep, and ask them what they might need in the way of a good prep job. And inspect the car, in the sun, when you pick it up.
'67 6 cyl Convertible, Clearwater Aqua