Here are a couple links about spindles/vfd’s that might interest you:
I have a 800w air cooled spindle that runs off a 2.2kw vfd (with speed control potentiometer). I bought the 2.2kw VFD from www.automationtechnologiesinc.com for $180, and the spindle I bought off Ebay for $135 (some suppliers ship from US). With those two purchases, as well as buying the wires/220v plug for the vfd, it comes close $400, so it’s a significant upgrade. To me, it is well worth it: 1) it’s a lot quieter than my old craftsman router that I used previously, 2) I can control the RPM’s via the speed control knob on the vfd, 3) it’s designed to use on CNC’s, so running them for hours and hours won’t hurt it.