My feeling (having converted a 7x12) is that you should get the biggest, least featured, cheapest, most rigid lathe you can.
Most of the discussion of the various flavours of 9x and 8x lathes centers around the half-norton gearbox, wobbly compound, cross slide feed slot burrs, change wheels etc.
When you do a CNC conversion, all this stuff ends up at the tip (actually collecting dust in a box under the bench
There are a few really stripped down models of 9x out there, and that would be what I would go for. Aim for one with a t slotted cross slide. From what hve gathered all the chinese 8x and 9x lathes are decendants of the Austrian Emco's which were 7x lathes. The 9X has quite a tall headstock and is thus less rigid.
Toss the single phase chinese motor and put on a treadmill motor and controller or three phase and VFD on the spindle.
Toss the leadscrew and mount a ball screw. Toss the apron and hang the ball nut on a plate.
Toss the compound, make up a ball screw drive for the cross slide. Mount a Phase II AXA to a plynthe. Build an enclosure. Add an spindle encoder. Run TurboCNC on an old Pentium or celeron, will be up and running.
It was just a shame I don't have space for a 9x lathe, as I end up torturing the 7x12.