Greg,
The louvered doors are a great idea for ventilation. I think you'll like that. Structurally, your stand should be fine. If it were mine, I probably would have used a few more 2x4s, but overkill is the name of the game if you look at my system closely.
Do you plan to have your prop tank connected to the main system? My only concern is the amount of heat that amount of lighting is going to put into the system. Leave some room in your budget for a chiller just in case...I know I ended up with one after just a few months.
In answer to your questions from my thread, my 250 is 72x30x27 (27 deep). Allowing for acrylic thickness and the overflow, the true tank area is more like 62x28x25. I'm finding that 4 T5s is plenty of light over that area. I have 3 250w DE halides, but I don't even make full use of them. To reduce the load on the chiller, save electricity, etc, I was running only 2 of the halides for 4 hours a day. I reduced that to 2 hours at some point, with no negative impact. Then one of my sockets started acting up, so I switched to the center halide only, still running just 2 hours a day. In all honesty, my corals have never looked better. The halide is ~10" off the water, and I running it mainly for the glitter effect. In the future, I will probably modify the lighting for 2 halides on a permanent basis, and put the other pendant over my coral QT / frag tank.