There are several low cost solutions, but at minimum you'll need a triple crank, BB, and probably a FD.
I've had great success buying a donor rigid frame mtb for cheap and transplanting the drive-train. Not "Classic" looking, but gets the job done just fine.
For example, this drive-train came off a $20 mtb garage sale bike, and bolted right on to a nice Centurion frame set I had laying around: