While there are very large studio camera stands available (Foba makes several, and Manfrotto serves the lighter end of the market), none of the commercially-available stands will fit your specs (and the closest fit is about $5K). But there's not a lot to building one unless you're overly concerned with super-smooth adjustments.
A studio stand consists of a large vertical pipe (3-4" ouside diameter) anchored in a massive base. Typically, the base is a substantial chunk of cast iron, but there's no law that says it must be. It simply needs to be strong enough to support the vertical pipe and the arm/camera combination, and heavy enough (and low enough, of course) to keep the whole thing from tipping over. A competent welder could easily build a triangulated base out of heavy tubing, and you can use ballast bins to bring the weight up to where it needs to be. At the top of the vertical pipe, you need a pulley. A counterweight attached to the arm carrier by a cable will run inside the vertical pipe, which will allow you to adjust the height with minimal effort, and make it easier to lock the height adjustment with a hand-tightened clamping system. (In the "real deal", you'd want the pulley assembly on a swivel so that you can pivot the arm around the stand freely, but one can make sacrifices for cheapness in the DIY-ish world and, I dunno, maybe pivot the stand when the adjustment gets out of the cable's range of movement.) And you'll need casters for the base ('cos it'll be a heavy damned thing to shift without 'em) and screw-down "legs" (really just friction points, common bolts with a handscrew will do) to lock it into place once you get there.
The arm itself is just another chunk of tubing. It might be preferable to use square tubing for a DIY version, since you can control orientation without having a key rail (a little strip that sticks out of the main pipe to keep it from twisting around). It'll need to have holes at one end for a 3/8"-16 bolt (to mount the head), and a way to attach a counterweight/stop at the other end. 2" tubing should work fine for that; it will give you more than enough excess capacity.
The only tricky bit (for some value of "tricky") is the arm carrier. That piece is essentially just two holes at right angles to each other. It has to be strong enough to handle whatever torque is applied (with some excess capacity to account for clumsy photographers), long enough along each of the tubes to mimimize torque-induced friction, and needs to have screws to lock it in place on both the arm and the vertical tube. Bearings used as wheels will make things a lot smoother and easier to adjust. And if desired, you can add crank adjustability both vertically and horizontally (easy enough with cables and capstans, and much cheaper than rack-and-pinion).
It's not strictly a DIY project unless you have some metalworking and welding skills, but it's not beyond most people who have those skills either. The materials are relatively cheap; it's the time and skill that goes into making it that may add up. But it's hard to imagine it coming in at anything near the price of the close-but-not-quite Foba.
