If your doing work that is serious enough to require a spindle you should really have adequate 240v service available in your shop. Even if this is only a hobby, or part time business.
If you really wanted to go ahead with something like this I would seek out the advice of ShopBot and or the supplier of the spindle regarding the compatibility of the VFD with the chosen spindle. I would only go ahead with this with their blessings. I would also make sure that this would not void any warrantees. You don't want to take any risks with this sort of equipment.
And one more thing. 99% of the electrical advice you read on the internet should be taken with a grain of salt.
Michael Schwartz - Waitsfield VT
Shopbot prs standard 48x96. Aspire. SB Link.