If you buy the right machine, used, you could care less about the parts you have to scrap to make it into cnc. Perhaps even, you could sell off what you scrap, if it is one of the more popular machines.
The seperate headstock of a real lathe is not something that you are going to find all that easily, most likely, its going to come on a bed, etc.
I suppose you could even cabbage a set of linear ways on the sides of an old lathe bed, so that you could make use of the existing spindle and tailstock.
I think you would be far ahead to start with a used cnc lathe, and retrofit a new control on it. Consider the cost of two sets of linear ways, two ballscrews, high precision spindle bearings, a full cnc retrofit kit, building a carriage, tool turret, and a thousand and one other knick knacks. There is just no payback in starting from scratch.
Check out the machines at www.machinetools.com, if you are having a struggle finding something.