I have a craftex CT129 mini mill. It's quite underpowered. Good enough for drilling holes in steel and aluminum, for milling, the DOC is limited to perhaps 1mm max at a time for best results (in aluminum). Not at all suited for milling steel. Drilling holes in steel, no problem, I often use it to drill 1/2" holes in steel.
There are a few problems with my mill. The taper is MT3 I believe, and I bought a set of large jaw adjustable vise grips and added rubber strips over the teeth so I can clamp down on the collet housing. I need to do this when tightening the draw bar to get it tight enough otherwise the bit will walk during use. That's a real pain when you are trying to mill something flat and you find out your bit is slowly pulling it's self out of it's collet.
The electronics box that is attached to the mill, it doesn't like vibration. I damaged mine from vibration, got it working again, but the RPM readout still doesn't work. If you buy one of these mills, the first thing I would do is to take off the electronics box, rewire it with longer wires, and mount the box on the wall, and not the machine.
How much space do you have? Do you have a garage or a workshop? IMO, you may be better off looking for a larger used mill, perhaps a bridgeport or something 220V. They can often be found cheap, perhaps cheaper than you would pay for a new one of those mills.
Personally, I would want an upgraded motor, perhaps from a treadmill, and a power drawbar upgrade for a CNC mill. The little mill I have, I use it to make parts for my builds, but it hasn't been CNC converted.
The CT129 I have has a ton of backlash. You can feel it in X and Y the wheels. Around 0.02" (a bit more on one axis and a bit less on the other). When I'm drilling holes and I want them to be aligned, I tighten the little screw handles and then reverse direction on the handle until the backlash is gone, then I rezero the dial for the handle and loosen the screws. After that I can count my rotations to get the next hole perfectly spaced. I've gotten used to it so I know exactly how much I need to rotate the handles to take out the backlash when reversing directions so I don't even need to tighten the screw handles. If you have the chance to look at one of these in the show room, you will see what I mean, rotate the handle then reverse direction, and for the first little bit of movement there will be some slop where nothing moves except the handle.
For a CNC you'd either need to get an anti backlash nut for the lead screw or convert it to ballscrews.
Some people use the G0704 mill, that's another one to consider.
Personally, I'd be looking for a used mill that is a bit bigger / more powerful to do a conversion on.