I've had a used chassis done, but it was in very good condition.
It'll need to be shot blasted and acid dipped which will clean up the outside, but that won't take paint/rust off the inside of the rails. The zinc won't bond with paint or rust, needs to be clean steel.
It comes down to how good your chassis is - the company doing the work should be able to advise you, but if its in any way crusty I wouldn't bother personally.