"If I buy an inexpensive manual square colum just how tough is it to convert to CNC and is it worth it? Is it possible to get somebody else's project for a good price?"
If you are comfortable with engines and machining then a conversion shouldn't be over your head, though the electrical and electronic parts might be unfamiliar and require some head-scratching.
All in the cost of an RF-45 conversion seems to be around $5k, though you will find people who have spent more and less. A Tormach or IH setup will likely run at least double that, while with a knee mill really depends on luck and shopping abilities, but this is a road fraught with dangers for the uninitiated.
As for buying someone else's project, your best bet there is to look around locally since you're not going to want to buy and ship something like that sight unseen. I don't see very many such projects on the market in my area, and when they do people want too much money for them.