I'd recommend against it. The effort of converting a manual mill to CNC isn't trivial, and starting with a round column mill means you're going to end up with a white elephant when you're done. Get the mill if it's a good deal, and maybe even mount some stepper motors on the X&Y axis with belts just to play around and use them as power feeds. Here's the deal with converting a mill to CNC and achieving reasonable accuracy and ease of use:
1. Computer
2. Breakout board
3. Motion control board, optional but highly recommended
4. Spindle control, might be part of the BOB, might not. Quite a bit depends on the motor and existing control. Do you really want to fiddle with those step pulleys every time you drill a hole and then switch to an end mill? Tool changes could take 5-15 minutes (or longer if you have to raise/lower the head). Yuck.
5. Electrical/electronics misc: E-stop, limit or home switches, wiring, enclosure, stepper/servo drives and power supplies.
All of the above are going to happen regardless if you start with a little Sherline, that RF30 clone you're looking at, or a G0704 or RF45. This doesn't touch the mechanical parts:
6. Axis motor mounts & drive (pulleys or direct drive)
7. Quill drive (really now. How rigid is the quill when it's extended because you don't want to move the head?)
8. Ball screws, nuts, mounts. (not trivial. Requires some machining and fitting, and possibly modifications to the table and saddle)
All that work and you still have a round column mill-drill. The parts costs will be similar to a like-sized square column mill, and the installation & troubleshooting effort will be the same or worse. Even if you keep the acme screws and just do a 2-axis CNC with manual Z (quill or head), items 1-6 above still have to happen.
My advice, having gone through this over the past year and doing countless hours of research before I plopped down a dime, is to buy something that will be:
A) useful when complete, and not abnormally irritating. I want a pleasurable hobby, not a psychotic heroine-addict mistress of a machine tool.
B) possibly worth something to someone else if I decide to abandon the hobby or upgrade to a grown-up VMC.
If you're spending the $ and effort to convert, why not start with a mill that has much greater potential when finished? It's like deciding to go racing and planning the project around a minivan.
Buy the mill, but plan to use it to to help convert a G0704 or something similar. And then sell it or keep it as a spare drill press in the corner like most of the Bridgeports I see in job shops.
And while you're fiddling with the manual mill and shopping for a square column mill to convert, do yourself a huge favor and start learning CAD & CAM now, rather than when you have a CNC mill lurking in your shop that you can't really figure out how to drive.
Even better - convert your lathe to CNC and watch it squirt out AR15 barrels while you count the money. Then splurge on a turn-key CNC mill that your lathe paid for and start machining custom USPSA open-class 2011 slides to fools with more money than sense at $500 a pop.
-S