I have a Mechatron spindle HFS-8022-24-ER20. It is water cooled for better low rpm performance. It is driven with a Hitachi WJ200 VFD. My spindle was bought with the power supply cable as an extra. The quality of the cable and connector Mechatron provided is excellent. Their cooling water connections are also quite good.
I did some significant research before choosing this as my solution. I could not get adequate detail from any suppliers of Chinese spindles, including US resellers. My view of the Chinese VFDs is unfavorable when compared to the Hitachi offering.
The Hitachi offers a multitude of configuration options using intelligent inputs and outputs. As Martin also pointed out, you also get SVC.
My setup works perfectly with the OF controller via the ModBus. The VFD is configured/outfitted to turn the dust collector and the water cooler on/off with spindle operation. It is also easy to delay the shutdown of the water cooler if so desired. I also have it configured to allow a pause in spindle operation if you want to pause a program.
The Hitachi manual(s) are a bit complex but make sense after some study. I use the Quick Reference Guide as well as the WJ200 Manual. I have also found Hitachi Technical Service to be excellent. They have gotten back to me in a timely fashion if I had to leave a message and their technical competency is unquestionable. I have always felt very satisfied after a conversation with them. Full disclosure, they did ask if I were trying to run a Chinese spindle. They seem to have a cautious position on trying to help with that.
I already owned another Hitachi WJ200 which drives a variable speed metal lathe. It has performed flawlessly. My experience with it supported buying another one for the Onefinity.
BTW, I am not trying to trash Chinese spindles and VFDs. They are quite economical and many folks seem to use them with great success.
Have fun with your studies!