Switching to a standing desk was the best change I ever made in my life of using computers--even more so than leaving Windows for Linux. It takes some getting used to, as I'm sure you've already heard. Even after a year or so of daily use, I still take sitting breaks every two hours or so. When I don't feel like taking a break, though, I can go up to ten hours with no problems. Since switching to a standing desk, I have received compliments on my erect posture... People who sit at computers all day tend to hunch forward, I guess.
As you get used to standing, your feet will change position, shifting weight from one to the other or balancing weight in between, without you even noticing. I find my knees rocking back and forth, practically dancing, with the right music in the background... As I typed that sentence, I realized that I was doing just that. Anyway, movement is good. I've yet to make the jump to treadmill computing, but my legs make up for it on their own accord. It didn't come naturally at first; I had to tell myself to move. Now, it comes naturally.
Also, when you're simply viewing the screen for any period of time, it helps to free your hands from the keys and mouse.
As far as the desk itself goes, you might want to look into steel wire shelfing. A really good brand in North America is called InterMetro. A lot of their stuff is industrial-grade. I use a Japanese-made version that includes a slide-out shelf for the keyboard and is rated to hold 135 kg per shelf. It's solid and dependable. The whole set-up only set me back a couple hundred bucks. If you're looking for something cheaper, though, like Fox says, you could smelt your own steel shelving from iron ore.