Hi. I bought my 5HP SJ300 new on eBay for $300. There's a 1 HP version
now, used, for ~$120. Too bad the encoder board
is more than the drive itself, but it is also available on eBay.
I fought myself over what to do with the interface. On the one hand, you do get more flexibility by intercepting the pedal signals and massaging before sending to the drive. On the other hand, it might be more reliable to connect the pedals directly to the drive and use the micro to command max current and voltage levels. If the micro were to fail, the drive would still work. It would also reduce the chance of sudden max throttle or brakes if something were to go wrong with the micro. But I also have a counter argument for that so you can see I've gone nowhere with this.
I did build a prototype to link the pedals to the drive and it worked well with the exception of proportional brake regen as mentioned above - the drive shuts down when you command 'stop' so it loses track of the motor speed. This will probably be cured by adding the encoder.
The SJ300, and I imagine many more also, can resume motor speed realtime without the need to stop the motor like some old drives did. Hitachi also provides free software
for their drives. I did try ProDrive package but never got it to talk to the VFD. IIRC, it had something to do with the drive port I was using and not the RS485 i/f. Allen Bradley, Siemens, ABB and Eurotherm come to mind as good drives also.