No brainer to me buy the frame.!!

You won't make it for less by the time you factor in machine time for grinding surfaces etc. Then you have all the time invested which will be many many weeks. With this you'll be up and working in fraction of time and making money.!

Then you have things like Class 3 ballscrews these shouldn't be confused with the cheap Class7 we buy. If indeed they are Class 3 then they will cost much more and $1500 would just about cover the screws and not rails as well like Boyan suggests.

BUT all this said if your going to this trouble have you thought about buying a second hand VMC. You can often find second hand VMC with dead or absolete Controllers that are perfect for conversion. Even with older ones that have many hours use and worn the structure is still perfectly fine and substantial.
Change the worn parts for new refit with new controller. Often you can reuse servo's and Scales etc even if they are analog drives etc.

You'll get a much better professinal machine for less or same money you'll spend doing this to this level and when done it will be worth double you invested.!!