Honestly? I'd hand make a Wooden container for the PC Tower, and coat it in about 5 layers of gloss enamal paint in your desired color, or a Plastipaint spray... 3 coats maybe.
THE CASE BOX
If you need an idea on how to construct it, think of it like an over sized bread box for your PC tower instead of a loaf of bread... you want the air intake to be coming from under the case, and a single flow out of the top/back area of the case... The reasoning is that greasey particals that tend to manage to coat everything around wont make it from outside the case, thru the fan, and from with in that enclosure to the inside of the case thru its ventilation. It can be nearly garanteed it wont even get to the outside fan if you construct it right.
CASE CHOICE
To assist in this, you can use one of those flatbed cases for yoru PC, as you wont be placing a Lot of tech in a computer for this purpose. It will save you a lot of room, and the cords you will need to run out of the case will be much more closely spaced allowing you to bundle them easier to avoid any mishaps in the kitchen.
CORD MANAGEMENT
When it comes to the cords going out of the box, I'd bundle them with in a plastic sheath and run that all the way to the wall to protect them from any spills/impliments/clumsy staff etc.eletrical tape the end of plug to the sheath and secure with a good 20 cms of binding up the sheath, to ensure the thick part of the plug sits nice and flush in teh end of the sheath. You dont want to rush this part because nothign is worse then a custom job you have to mess every month due to shoddy application.
DISPLAY PROTECTION
Similar concept but use a strong clear perspex for the front panel of this box. I would suggest using TWO front pannels, one permanently afixed, and the 2nd that should slide up for removal should be an inch larger in all directions. This will keep internals clean even when your washing down the external perspex front panel.
PERSONAL PREFERENCES - if you hate this idea quit reading and save yoru eyes.
I PERSONALLY, would have a slightly larger box that held the flatbed case, the display, and a surge protected power board in it. The only cord coming out of the box would be the Power Boards cord and the networking cord, each individually sheathed incase you ever have to move the Unit.
The case's side pannels would extend atleast 6 inches lower then the base of the container to raise it off the ground, allowing for the fans to have sufficient room to feed air up into the box. These fans would be externally set with air filters that come stocko with most fans. The same for the Outtake Fan at the top/rear. Air Filters can be purchased and replaced if they ever get unrecoverably greasey. The restaurants petty cash on any given day would cover as many filters as you'd use in 100 years.
The front panel would extend down till there was only 2 inches of free space at the very front of the "air space" so that you get a downwards momentum on the grease particals, promoting them to hit the base and never lift into the intake fan in the first place.
Also ensure the overal base area is sturdy, as you know, kitchens can be busy and dangerous. Under no circumstances should this be able to be knocked over, and even a flying pot of angry half cooked crab shouldnt be able to damage it/penetrate the shell.
Your access panel (the removable panel to get inside and fiddle with it if you ever HAVE to) should CERTAINLY be at the rear, as reloading the case will be easiest by placing the monitor in, then the box, then the power board, and connecting all connections, before closing it up nice and tight again. You may consider having a slide in panel like the front panel to totally seal this thing up. I'd just use soft flexible thin rubber wings that push into place when you slide the "access panel" on tho. what ever you think is most effective.
THE MOST IMPORTANT THING
The main concern you should really have tho, is how big is your order display? can you run it at low res on a decent size screen to make it READILY readable? does the kitchen need ACCESS to the computers function? or is there a better way to clear off completed orders? Such as reliable notification to the person who placed the order into the PC via the front desk? You really need to CLARIFY your needs in total completeness..... (crap sentence). Anyway, let me knwo what you think, and redifine what you need ti to do, adn I'll see if Ic an get something simpler but effective if you want.