In my experience, you will want the dimensions of absolutely everything. Buy your casters and burners and take them with you to the welder. To make sure your dimensions are right for your equipment, I suggest laying out masking tape on the floor and putting your kettles on it to make sure it will work.
Build it large enough to go two batch sizes larger than you ever thing you will go, but still hold the kettles for the batch size where you are now.
I looks like you are doing a cooler shelf. If that's the case, put some heat shields on there. You will melt it in no time having it that close to the burner.
For your bottom tier, you need to decide how you're going to get the beer to the fermenter. You won't be able to gravity feed from 10" off the ground. When I did this and overlooked that detail, I used a pump. If you are using a pump anyway, I'd suggest going single tier. It gets old lifting the water up to the top tier of a gravity system. I have done it both ways, and while gravity is infinitely reliable, the single tier has lots of advantages.
If you are worried about space, consider going for a folding shelf setup for the second tier. If you go look at the pictures of my current stand and go back in the google album and look at my last stand, I had two folding shelves, one of which held the mashtun/cooler and the other that was a work station. The whole stand stored in a 22"x22" space.
If you have a pump, get the welder to make a mount for it and a drip shield. On your burner wind screens, make sure you leave them open on one side. That way, you'll have a way to have your gas plumbing go to the burner and, also, you want there to be a way for fresh air to get in between the kettle and the burner for good combustion.
Get them to do everything you can think of the first time. It is always more expensive to go back and add stuff later (ask me how I know). With that in mind, if you ever think it is the slightest possibility that you will go to a kettle or keggle mash tun, get a heat shield and pot support put on that middle tier. It makes it so much easier to add the burner if you go that route. I never thought I would do it and spent extra when I changed my mind.
I have had two stands built by others based on pretty basic sketches. My guys want me to supply all the details because they don't want to get blamed (based on experience dealing with others) if it doesn't end up right for some reason.
A work shelf is also a handy thing to have. I didn't put one on my most recent stand and miss it. Also, have them put some pegs to hold spoons/mash paddles.
Finally, when in doubt, build it stout. Go big and robust. If anything, it makes it easier to mount stuff to it later, like pump switches switches, control panels, and gas plumbing.
Good luck. And keep the pictures coming.