MP,

Dont buy ready made nuts, they are expensive. buy an extra 100mm of the rod and make a tap as Jonathan has suggested and cut the threads in some delrin (engineering plastic). It will do for what you want. You don't need to harden the tap for plastic.

As to the steel frame, its massive overkill for a first tryout machine. Also those motors/driver wont be up to the job. My recommendation is keep it small and simple, you can do a lot on a small simple machine as long as you plan carefully and dont expect miracles.