OK I have actually run EXL Sheldons.
3/4hp is fine..
A 10 in IS NOT a 13 in lathe... A Sheldon R series, is a serious lathe.. Not a light school/homeshop lathe like the smaller Sheldons..
As to VFD rated motors... If motor is in good condition, and made from say early 50's on.. It will handle a VFD just fine..
I would not put a VFD, on an old cotton wire insulated motor.
VFD rated means.. Good insulation. And ability to run at very low rpm for a very long time.. It can handle the extra heat build up..
Since the bed on that 10 in lathe, is somewhat less than say 30 ft.. It will never have much of a continuous full load at low rpm...
A quick check of motor housing temperature with your hand, will tell you if things are getting too hot.
NO need to put a bigger motor on the lathe, You will still have to change belt positions once in a while, AND use backgears...
A direct drive motor to spindle, would require a larger motor to still provide usable torque at low RPM. Not a problem with stock countershaft and backgears...
Carbide is just OK on a light lathe, I would use HSS most of the time, saving carbide for... Abrasive materials, Hard materials.
I do a LOT of low speed threading to a shoulder, Carbide has no advantage over HSS there.. A quick touch up with a stone, keeps the HSS razor sharp...