Dont know about your plasma cutter, so i"ll pass (I,m Hypertherm fan)
Rack and pinion, yes
good quality tooth rubber belt, yes
Your bike chain will wear, you’ll end with slack
side screws are too slow for thin stuff.
YES, you need a Z axis, yes it will go up and down on every pierce.
No, you dont absolutely need torch height control during cut (THC) since your table is small.
I have just built a small table and suffered many headaches for weeks because I was trying to use a cheap pilot arc plasma system on it. It just would not work and fried electronics, never made a cut with it.
In the end I stumped up a lot of cash for the smallest hypertherm unit, a 30xp hand system. Since fitting it, I have not had a single hiccup it’s been running outstandingly.
As for compressors, any unit which can output the required CFM will work.
You can save yourself a lot of grief by getting the right plasma unit, I wish I did earlier!
That grating will work but it would better to have a spike bed as the flame gets upset when it crosses a slat underneath causing rough cuts etc.