So let's just say that I was going to use threaded bar I know it's not ideal and it won't be as accurate as lead screws but it's the way I'm going for mk1.
That's unlikely to work with steel nuts, the friction will be too high, unless the spring is very weak and, since spring force should be much greater that your cutting force, that will limit the machines ability somewhat ;)

For a simple cheap job use a stainless steel rolled treaded rod with a tight delrin (or PTFE) nut (I think Irving has a description of this somewhere on this forum).

Bill