Find out how they're going to do it. I did my own, and dropped the belly pan, which means pulling off the axles, disconnecting the electric brakes, and then a small bit of rewiring as the wires needed to go around the tank, not over or under (or through) it. I think some shops might just cut the belly pan, and who knows how they might plumb it.
For that price, they should be doing the full monty belly pan like I listed above. Are you going to get a separate valve for the gray, or are they going to try to give you a single hookup? (single makes things easier, but not completely necessary).
I also put in tank washers when I had the tanks available. It's a cheap part and easy to do when doing the tanks initially - just make sure you winterize that if needed.