Thanks Bob. I was doing some more reading and found the recommendation
of 2X max load being a better minimum size for the idle (converter)
motor. I've trying to find a 40 HP 220/240V 16 pole (450 RPM) motor
(or an 8 pole/900 RPM if I can't find the slower). Is it possible for
me to oversize the starting capacitors enough to avoid the pony motor?
If not, any suggestions on how to automatically engage the pony during
startup and then disengage it once stated?

Also, I was considering powering a small fan to cool down the enclosure
and motor. If I use the idle motor to power the fan, is that going to
be a problem so long as I keep the blade size small (e.g. 3" - 6")? I
was thinking of rigging up some kind of furnace filter type setup to
allow air to flow into the enclosure; I'd place some kind of metal
protection that allows the filtered air to pass through, but still keep
hands away from reaching inside when the filter is removed.

It seems that it will be more cost effective to make a nice 3 phase
converter even if I decide to go with a VFD later on. It seems that
oversizing a 3 phase converter is much cheaper than oversizing several
3 phase VFD's to run on 1 phase power. This is of interest to me
because I could see myself adding a VFD to several machines... I really
don't want to have a single VFD running the whole shop--it's a
convenience thing.

Please let me know if my cost analysis is incorrect. Thanks.