Your lathe has a 7.5 HP motor, so your minimum Idler (RPC) motor size will need to be 7.5 hp. No need to run a giant RPC, the cost of operation is fairly low for what it provides.
Static phase converters are not (at least back then) that great as they convert three phase motors to single phase resulting in loss of power and run hot.
VFD's provide the ultimate control and options but as you know come with a price for larger units.
I have been using RPC (balanced) for 15 years now. The current set up is wired in conduit thoughout the shop and provides power to nine machines. I'm a one man shop, so running more than one machine at a time is not common, however I have run (no load) two mills, lathe, cold saw, band saw, iron worker at the same time with excellent balanced (+/- 3 volts) readings. In essence, each machine becomes a RPC for the next (I believe).
Check out Phase Craft on ebay, I purchased his control panel for a 7.5 HP Idler for $200 free shipping. It came prewired and balanced. The motor I used is the same motor from my old shop's RPC and runs quiet and efficient. You send him the information on your idler (size,RPM,service factor.etc) and he builds the panel accordingly and balances the legs.
As you can see from the photos, the machines are decent size and run fine on a 7.5 HP RPC. The largest machine (iron worker) is a 5 HP unit. The RPC is on a 40 amp circuit on a 200 amp sub panel.
I recently added a VFD (with the help from a fellow member MJ) to the plate roller for more control. The VFD allows for POT, hour meter, ramp up/down, braking, as well as torque values to be adjusted. This VFD does not convert the power to 3 phase in this case as I use the RPC for that.
I hope this helps.