As usual everything is more complicated in Daniels world.
Yes there are minimum sharges.....it is actually closer to $350 around here (500lb). I do large batches that generally run 2k or more. If I don't have enough (rare) it's not usually that hard to split it with someone.
Vent holes are not just for expansion. If you dont have them some parts will actually float. You also need to place the holes so that they will let the zinc in AND back out or you end up with a steel coated zinc ingot.
New work only requires very minimal venting (1/4" holes.)
Old work that may have rust inside will require large holes to allow the acid bath to remove all the junk.
A good shop understands this and I have yet to have these staining issues you mention. I have work in the shop now that has been driven in the NE winters for a decade and has yet to bleed.
The only unusual prep my PC shop does is warm water wash and rub down with scotchbright. You can hardly break the surface with a hammer, and in general I'd have to say it is better than going over bare steel.