Hi Monty,
All good advice from Daz, and I think he has put the cost issue nicely in proportion. My thinking tends towards the practical use of the machine. I have nothing against building a machine for fun, but would say the end-purpose of such a machine is to use it to build something, so why spend a lot of money on converting a manual machine with all its possible pitfalls?. As Daz says, things like ballscrews can be problematic, and if they let you down, then its virtually a complete loss, but on a complete machine, you can go back to the manufacturer. This is why I suggest get a complete machine, and spend what money you have left on making something with it. I don't know how much experience you have with this kind of machine, but I found that it was only after some time did I understand what I actually needed the machines to do, which led to subsequent modifications.

You mention an 'entry-level' machine - i assume by this you mean something fairly simple and cheap. In general this means the range of 'chinese-type' clones these days, such as the Sieg series - all reasonable machines for the price, but they will be costly to upgrade to CNC, because you will probably have to change most things except the main castings. Again Daz has indicated the scale of this. But its your choice at the end of the day - just have fun doing it.