Consider building a horizontal "V" cradle fixture with ball bearings where the pipe will lay in the "V". Rotate the pipe with a motor driven belt or chain drive at a low rpm. Mount an abrasive cut-off saw on a platform that is mounted above the "V" cradle to do the cutting. You would only need a low cost 10" or 12" cut-off saw for this, and make a deflector box for the spark shower if needed. You could also cut culvert pipe with a diamond masonry wheel. This should be a relatively low cost to build.
If you don't want the spark showers made by a cut-off saw, mount the upper half of a metal cutting bandsaw on the platform instead of the cut-off saw. The bandsaw throat will need to be wide enough to cut through the wall thickness but will not need to be the full 30" diameter.