You're not going to like my suggestion because it fails most of your criteria but will work wonderfully. It worked for me for over 2 years.
That suggestion is to use a Silverstone FT02 rotated case and remove the fans. Then use a passive PSU and Nofan passive CPU cooler. The gentle updraft from the CPU cooler and the PSU will prevent dust ingress, but will not be sufficient to suck dust in.
BTW with regard to the room being dusty, have you washed the room - floor, walls, and ceiling?
That suggestion is to use a Silverstone FT02 rotated case and remove the fans. Then use a passive PSU and Nofan passive CPU cooler. The gentle updraft from the CPU cooler and the PSU will prevent dust ingress, but will not be sufficient to suck dust in.
BTW with regard to the room being dusty, have you washed the room - floor, walls, and ceiling?