Dave has a good point, Steve has not stated what he wants to do with the CNC, nor what his constraints are (budget, space, power, opportunities, etc). I suppose there is "no free lunch". If you buy a cheap mill, you'll end up with a cheap mill - and perhaps that will work for you. You can convert a manual machine / build your own - you'll put in lots of time, and it may work out great.
I have just started down the CNC path with a 20 year old industrial machine (a BP VMC 1000). My all in cost will be about $12,000 CAD ($6K for the machine, $2.5K to move it, and additional to sort out the power) for a respectable size machine (39"x20"x16"), full enclosure, 20 tool ATC, factory 4th axis, chip conveyor, tool blow off, air padded spindle seal, 40 tool holders - basically all the stuff. I get it that the software can be another whole issue - but that is true for about any machine. This runs a pretty standard G-Code (a DX-32 controller).
Even the largest, fully loaded Tormach (or equivalent) is still less abilities than what I got for less than half the price. I'm sure Dave is right that I won't be able to use it's fullest abilities. If I got half way there - I'd be pretty happy.
Still, this path is "no free lunch" either. I've done a lot of cleaning, fixed numerous minor little things. I saw the machine run, but I have no doubt that there will be more little issues to sort out. I have more risk on how much $$ I'll spend compared to a new machine - perhaps a critical component will fail (many parts are standard electrical components, quite a few parts are also available from BP or various 3rd party vendors - but certain failures will render this a boat anchor) that is part of what makes an old industrial machine so cheap. You have to factor in the risk on your money. Perhaps I've got some expensive scrap metal?
Per the other comments in this thread, I'm hanging onto the manual mills. I got them cheap, and now cleaned up, tool'd and running - would probably fetch even less! Several years ago I added onto the shop, so now I've got room for the VMC.
David C. Lathes: Enterprise 1550, Smart & Brown 1024, Mills: Maho MH 600 x2, Cinci Toolmaster, Kasto hacksaw PSB 210, Grinders: Cinc#2 T&C, Ingar 618 SG all but the first came as projects, now they support an endless list of more projects. Good thing I have a day job!