Migrating from Mach 3

[A_Camera]

I agree, you shouldn't carry on with XP, both W7 and W10 are better but if you want to carry on using Mach3 with parallel ports then you are limited to W7 32 bits, but if you want a better PC and a motion controller then you can also use W10 64 bits. All of the UC motion controllers can be used with Mach3, you don't need UCCNC if it is not working for you.

[m_c]

Turn does limit choice, as it's a relatively small slice of the CNC market, compared to mills/routers/plasma cutters etc which all operate in a very similar way.

I'm pretty sure the UC motion controllers (or it might be Pokeys - I can remember a discussion over on the Yahoo Mach group a while back about multislot threading...), and the PMDX controllers support turn, but you'd have to use some flavour of Mach.
CS-Labs supports turn, but you have to use their fully featured controllers, along with an MPG module for threading IIRC, and again you're limited to Mach at the present time.
Smoothsteppers also support turn, but again are Mach only.
Dynomotion KFlop would give you the option of Mach3 or KMotionCNC (it's free), which is what I personally use, but it would be reliant on you adding an encoder for threading. I originally ran with a KFlop and Mach3, as KMotionCNC at the time didn't have much support for turn, but after yet another Mach3 glitch, I moved to KMotionCNC a few years ago, and it's been flawless (more than which can be said about the operator..).

Then you're into standalone controllers.

It really depends on how much you're willing to spend, and how much re-wiring you're willing to do.