I think that I can help you out a little with your HVAC situation as I did a similar setup to what you are proposing in my room. My theater is not connected to the home's HVAC system but instead exchanges air with the rest of the basement via two dead vents. I have one in the front that acts as the supply and another in the back for a return. The return is connected to an inline fan which draws air out of the room. The front is unpowered, but since the room is sealed up so tight from the soundproofing air is automatically drawn in due to the decreased pressure created by the return inline fan.
I got a little lost in your last post about how you were going to isolate your exhaust with the projector box, but I think that an air exchange system would work well in your situation.
Looking at some earlier pictures in your thread, it looks like you have some open space on the other side of your screen wall. I would build a dead vent in that place and have one or two supply vents in the front wall of your room. The other side of the dead vent can just be open to that adjacent space and it will draw air from there into the theater.
For the back I'd do a return with another dead vent but you'd have to find a place to put it. It looks like there isn't much empty space back there so it might take some creativity.
For your fan, I ended up using a similar one to the one you linked to. It was the 8" model though, which was available at my Home Depot. I used a reducer to attach the 6" flex duct to the fan. I'd recommend the 8" model since it wasn't much more money and has a higher cfm rating. Overall, I have been happy with my fan, but keep in mind that it is nothing special and there are much better products out there (of course they cost more though). I'd keep it accessible so that you can replace it if need be in the future.
I used a variable speed control meant for ceiling fans like this one:
It looks a little less industrial than the one you linked to. (The good ole' 1940's!
I have been happy with my setup so far and it keeps the room at a comfortable temperature even when I do a double feature with four people inside. You mentioned the fact that an air exchange setup such as this allows you to run the heat in the rest of the house without heating the theater, which is a nice feature. It's also good because it completely separates the theater from all that metal ducting which we all know transmits sound rather easily.
Some things for you to think about.