the fact that Mechatron (Shop) now sells the Hitachi S1 (standard series) as accompanying VFD comes from the fact that first the Omron MX2 had availability problems (they sold that one when I bought my spindle) and then the practically identical Hitachi WJ200 became unavailable too. Now they sell the Hitachi S1 instead.
Yes, someone in the forum owns one.
The Hitachi S1 (Hitachi S1 Series Basic Guide) seems to be nearly identical to the Galt G200 (Galt G200 Series Manual (PDF)). It differs from Omron MX2 / Hitachi WJ200 in its settings names and numbers, but its capabilities are comparable. It also has Sensorless Vector Control like the Omron MX2 (Omron MX2 User’s Manual) / Hitachi WJ200 (Hitachi WJ200 User’s Manual).
Should you buy a Hitachi S1 and it still can not be selected in the Onefinity Controller VFD list, nor the Galt G200, you can make the Onefinity Controller to know and run it this way.
You pay much more when you buy the VFD from Mechatron, what you pay for is that they will have set it up for you. What you could do is buy their complete control cabinets. Building up your control cabinet can be a little challenging to beginners, so their ready-to-use cabinets are fine (but also expensive). See here for more information on building your own control cabinet.
I did not buy my VFD from Mechatron, but from a local CNC equipment supplier (360ish € in 2021), I just bought the spindle, the cooling station, the coolant liquid, and the ER20 collets from Mechatron, but I have the knowledge to program my VFD and to build my control cabinet myself.
What I strongly recommend is to buy at least a ready-to-use spindle cable together with the spindle from the spindle manufacturer. It needs a Phoenix M17 connector and you need an expensive crimp tool to manufacture a spindle cable with that connector yourself.
When you buy a VFD I would in any case also buy the matching EMI input filter and a braking resistor. The specification with parts numbers can usually be found at the end of the VFD manual.
EDIT: My Omron MX2 2.2 kW VFD is now available again here, but it is more expensive now with 442€
A good VFD manufacturer that produces in the UK is Invertek. They have Sensorless Vector Control too. Also with this VFD you can find user experience in the forum