On my RF, I used a 2HP package from Dealers Electric. It is comprised of a 2HP VFD (FM100) and 2HP motor. I have found that I can get a speed range of about 300 to 3400 rpm out of the one belt setting (1800 rpm motor) over speeded to about 116 Hz.
I have had it to lower rpm, but there isn't enough torque to do anything. Matter of fact, I have had it down to about 10 rpm and with no load, it will trip out in a matter of minutes!
It has been running this way for a few years and I find it completely satisfactory. I don't ever run it above about 3500 rpm as the spindle gets quite warm after a period of time. Don't know for sure if it is due to the seal on the quill or the bearings. I have seen opinions on both.
Somewhere, I read the average "turn down" ratio is about 10:1 and my figures show that. So it doesn't matter if you use an 1800 and over speed it or if you use a 3600 at normal speed, your are still looking at about a 10:1 turndown! There are torque/HP tradeoffs for either type of installation. Look at the manual and you will have to make your own decision.
As for the rating of the VFD, "most" modern units are fully rated to about 3HP whether run from single or three phase. Again, check the data sheet and it will tell you.
By the way, next to cncing the machine, this was the best investment I made!