Someone wants $100 for a [supposedly] nice condition used 10 horse
motor... It is not exactly very cheap, but I can have it right now...

Its a time/money/hassel trade off.
Lost creek machine in Ottawa, IL sells used 3ph motors for
$10 per HP. Same price so its not out of site, but not cheap either.

Something to think about: You will need start and run caps. The
bigger your idler motor, the more caps you will need. If you are
buying the caps new, you might incure more cost for the caps than
your motor. I recommend that you figure out what you need before
you buy parts because you will spend more to build a 10HP converter
than a 5HP converter.