For that size lathe, I wouldn't personally got for anything less than 2HP for the spindle for turning.

For a 4th axis, that's where things get tricky. Even with a large servo, by the time you gear it to get 2-3000rpm, you end up with very little holding torque for doing accurate positioning work.

And not only that large servos are expensive. This is something I've often though about, and for ideal use, you need some way of changing servo:spindle ratios, so you can have one high speed low torque ratio, and one low speed high torque ratio. The problem is how to achieve that while minimising backlash.