I just assumed that since it has a 0 to 10000 rpm variable speed motor you could run it around the 300 rpm sweet spot, it would be tricky to actually tell what rpm it was turning at but a little trial and error should get you in the ballpark.
I don't think anybody would seriously consider connecting it via the flex shaft. Most people would remove the flex shaft and both grinding wheels and connect a lovejoy to the one of the 2 output shafts.
As far as power goes the original Molon AC motor I used worked okay and it was only .33 amps, the grinder is 1.3 amps so it might work fine, or it might not. I'm pretty happy with the DC motor so I won't be buying one to find out.