I started with a used machine (mostly just the frame), added steppers, wired the controls, and was delighted when it made first motions. I am still working on it, but i learn a little more about it with every bit of progress, and even every set-back. i am still not a master with CNC, but i learned a lot from doing, and researching what needed doing.
I think what you start with depends entirely on your skill-level, and your time-span to get it done (if ever). starting with a kit, will give you a bunch of pre-selected parts, and all you do is put it together. no real research needed.
Building from scratch is like a trial-by-fire. you will find a lot about the machines through researching what you need. but it can get expensive if you get it wrong.
so value is in research on what
you need your machine to do. what framework will be necessary to handle what you want your machine to do. and what motion components will be needed.
at least, that is what i have learned.
and there is always a chance i may be wrong.
~Travis