You said best then cheapest? which do you want?
Least expensive way? Wire switched single phase 220 to any two of the three input lines on the main motor. Spin the motor in the direction you want it to run by what ever means is cheapest for you. I have seen single phase motors belted up, lawn mower recoil starters adapted and used, electric drills. I have seen rope just wrapped around the shaft. I have seen one that had the fan shroud removed and they stuck a screwdriver in the fan and spun it up. You dont have to get it up to speed just going the right direction as long as there isn't any load dragging on it. By what ever means you choose get it spinning and then throw the switch. The motor will run and you will have near 2/3 name plate HP. 3 phase for the power feeds will come from the same single phase you just attached to the main motor and the third phase will come from the un-powered third input connection for the main motor, the voltage being induced in the main motor just like the idler motor in a rotary set up.
VFDS are great where they fit but have their own set of problems too. Old motors dont like them. I found that out after ruining an old U frame 1100 RPM 10HP motor with one. Motor shop guy said something about voltage spikes and old type insulation. They also ramp up quickly in price once you get past a couple HP. I don't hate VFDs, but they are not cure-alls. You can walk your way into a rotary by building it yourself. up to 10HP converters/starters are on Ebay right now for $99, look up Phase Craft, the are made in USA. Use it to start your mill and if you decide later you need some extra power then buy an idler motor.