rails and ball screws at the most will be 1500$
motor , spindle and so 2000$
500-700kg of steel 700$
mig wire, gas and so 200$
epoxy paint and so 200$
additional stuff 1000$
servo motors 1500$

That in worst case scenario, if you are careful with the design it will be more cheaper.

BUT:
-the machine would be exactly as you want it
-better built than the Chinese frame.
-Cheaper
-a lot of small things would be right, that makes a machine GREAT

Dont copy all,use it as guide lines only, build it from scratch and i assure you it will work. It seems you have done quite some research already. Plus one day next time you will like something like this , which you can buy or make your self. So why not from now plan how will fit and be prepared. The Chinese frame will not allow it without sacrificing something.