By going to a 3 phase motor, there are no start/stop issues to deal with. Using a VFD you get both controlled acceleration and deceleration to whatever degree you want. I have done a number of smaller lathe conversions, often go with a 2 Hp motor if it will fit. Alternatively you could go with a DC motor and controller with braking. A lot easier then adding clutches and brakes and the space it would take up. There are also brake motors for either single phase or 3 phase, but they are pricey. Expense wise, if you do both a mechanical brake and some form of clutch, a VFD and 3 phase motor would probably be similar in cost.
Slamming the lathe to a stop, I would be less concerned about the motor and more concerned of the longevity of the gear train/drive. There is a lot of momentum in the system, a mechanical brake slammed on at speed generates a lot of braking force and something would eventually break. VFDs you can control both the time an slope of the braking rate, so would be a bit gentler on the system. I normally use 3 second braking with VFDs, 1 second only for slow speeds or emergency braking.
