You could build your own VFD pump but last time I looked into it, it wasn't that much more cost effective than just buying a VS pump. You need an inverter duty motor, special sheilded power cables to prevent interference and to protect the drive and a NEMA water proof enclosure (if outside). I know of a few people who have tried this and skipped some of the steps only to have a drive fail in short order. BTW, you can sometimes find Intelliflo drives on eBay.

If you are concerned about the VS drives, which I don't blame you, you can get most of the energy costs savings with a two speed motor. That is the route I chose because when I did the economics, I found replacing a single speed motor with a two speed motor (off craigslist) would pay for itself in less than a year and it would take more than 5 years for the Intelliflo to be more cost effective than the two speed.