What hardware do I need, exactly

Wow, that looks like quite a rig. Would you mind detailing the rest of your setup?

There is not much more to it

I've tried to use every feature of the 7i76e so I can learn as much as I can about LinuxCNC and Mesa.

The relays at bottom left. 1 switches the plasma torch on and off, the other switches 5 volts to the stepper driver disable signals on estop. There are 2 bus bars between the 7i76e and the power supplies. These are +24v (on left) and -24v
There are 2 DB9 connectors underneath. One for home/limit Gantry sensors and the other is for the sensors on the table.

The stuff on the door is mostly mains power, mains breaker, estop, pause/resume switches and indicator lights. The 4 LED's on the right are (Plasma Arc OK and indicators to show power supply status (48v, 24v and 5v). This was suggested by a friend and it has been handy for trouble shooting. I've also added a couple of neon indicators for mains power status.

The large control box is for my external 433 mHz wireless pendant which is wired up to the 7i76e inputs.

Plasma is much more complex so it might be abit overkill.

Nice. What is the size of your cutting bed and what size motors are you using? You mentioned NEMA 23's and 34's earlier but what are they rated at? How many inch ounces? Or is it ounce-inches? Did you opt for threaded rods for the X and Y movement or rack and pinion? I have so many other questions, I hope you don't mind...

The X & Z axis are driven by 269 Oz Nema 23's that draw about 2.8 amps
The Joint axis Y uses NEMA 34's 1000 Oz/in limited to 3.5 amps
The Z is 5mm/rev ballscrew
X is rack and pinion with 3:1 drive I built
Y is helical rack and pinion with 5:1 drive I bought (hence the use of NEMA 34's as that was what the reduction drives were built for).
Remember that plasmas are fast and light with no cutting forces vs routers that are slower and heavier so my hardware might not suit your machine.

Huh. Thank you for the detail.

Plasmas have no resistance forces from dragging a cutting bit through material X, right. Plus they wouldn't need to be stiff enough to not deflect under those loads so the gantry is lighter in every sense of the word, too.

Well shoot, I was hoping to get all the electronics figured out so that I could start shopping for deals, but it sounds like I might actually need to build the bed, gantry, and get a spindle mounted in order to figure out actual weights and that et cetera first. Feh. What was it that general once said? Something about battle plans not surviving first contact with the enemy...