If you have a media blaster near you, give 'em a call and see what they'd charge.
If you're doing it yourself, there's several ways to go about it. Personally, I'd bring out the sandblaster on light pressure and hit it. (To the naysayers, I've done it for years, never warped a panel, and had success.)
Another course, as mentioned, would be the DA, with course grit paper (80 would be my preference) and get as much off as you can. Wear a dust mask. Rust dust will give a nasty head cold.
You can also use an angled die grinder with a purple rust buster on it and work into the bottom of the pits.
After that, hit it with a conversion coating, something like PPG's metal etch, rubbing hard with a red Scotch Brite pad working the conversion coating into the pits. Give a rinse and hit with the DA again, something along the lines of 180 grit.
Use a good primer. Something like a quality epoxy, or, my favorite, PPG's NCP line of primers.