Rick,
I was just throwing the 70 out as an example, it my personal favorite C&S locomotive of all time and second favorite, after the Uintah/SV 2-6-6-2s.
You are certainly correct about a minimum of three locomotives to spread out the cost of fabrication and construction.
I would think a minimum of three would be needed just to make the operation viable from an operational point of view.
For maximum revenue, I would think the operator would want two sets of equipment to run up the hill each day, maximizing capacity.
This would require two locomotives. A third as a spare would be necessary for days a locomotive had to be out of service for inspection, boiler washes, maintenance, or mechanical failure.
The 71-73, complete with "beartraps" would be awesome, even if they were modern oil burners.
Kevin