I found my reductions, rack and pinion with motiontek, and rails with linear motion. I believe both advertise here. Depending on what type of cutting your going to do, take time to shoot Tom an email to get help selecting your reductions. I think you will be happy with CandCNC products as well as the K2 Z axis, both were my choice and I can't say enough about the products. Take time to look at precision plasma products. They seem to have lots of knowledge.
I built my own system and many times wished I would have went with one of precision plasma systems. I personally created myself more work than necessary. Take time to read posts, you will find lots of info and designs that you can incorporate into your own build, and everyone seems to jump in and help.
Good luck and have fun with your build.