Do some more homework. Plasma needs high speeds compared to routers. Steppers lose torque at higher speeds. Rack and pinion ususally results in a gearing increase (eg a 1" pinion moves along the rack 3.14" per revolution). With high gearing you get plenty of speed but really low torque. Torque is what allows you to accelerate. It's like running a road race with lots of turns and corners in a car with one high gear and a 25hp motor. If it's geared for the top speeds of the straightaway it will take corners like a dump truck
You will get pretty poor speeds and performance with small motors (180oz-in are tiny by most standards for a bigger table) and the low voltage supply. That setup might be okay for a small desktop mill but not for a plasma cutter that needs to run a full shift.
You stated that you want to do it right but then selected the cheapest solutions you could find (?). There are dozens of questions about plasma cutting that you need to ask if you expect to be cutting 8 hrs a day. With the wrong setup you will spend more time and money replacing tips, recutting scrapped parts than actually cutting and making money.
Look at the commercial tables and realize that if they could get by with cheap components and stay in business they surely would.
You need to worry about things like: a constant cut gap, plasma friendly CAM software, pierce cycles, different pierce and cut heights, surface touch/detection, optimum feedrates, artwork generation (no, you can't load a picture (bitmap) and cut something no matter what the marketing DVD says!).
I see people buying 1200 oz-in motors (arg-arg) that are WAY overkill and others trying to use small under-sized ones. I think a 296 oz-in geared 3:1 down driving a rack and pinion is on the lower end of the target range for effective speed and torque. You can't just choose to cut at lower speeds if you can't get to the ones recommended since slower cutting vastly effects the cut quality.
Read the table build threads and look at the free table plans. I don't think you will find many using unipolar 180oz-in motor drives.
Also think noise avoidance (EMI, RFI) both radiated and conducted. The cards you listed do no appear to have any provision for input signals back from the table with any form of isolation. If your Plasma uses HF start then you have a level of noise to deal with that will leave you frustrated when the wrong interface components.
There is no "Z axis of choice". It's just another axis. If you want control of it for height you have to design and build it just like you would the X and Y (but maybe with a leadscrew instead of R & P). If you want control of the height while cutting you need to read about Torch Height Control (THC).
Then lastly there is the dust and smoke problem. Often overlooked, you will quickly discover that a plasma table will deposit a volcanic type dust (abrasive and conductive) in/on ANYTHING within 100 ft. Water trays help but are only about 25% effective. They do little for the smoke and dust from the cutting process. Your electronics will suffer, your lungs will suffer, and your computer will present you with blue screens.