Need help with stereo cabinet design.

If you're considering a built-in cabinet I'd suggest locating the fan in the attic to get the fan noise as far away from the equipment, and listening position, as possible. It can vent either directly to the attic or be ducted to the outside like a clothes dryer with a one-way flap. Although, if the latter you might get cold air blowback in cold weather even with the flap. Best choice of fans would be a squirrel-cage type, like the fan on home forced air furnaces; but smaller. The fan control can be located in the listening room for convenience. That type of fan is more efficient than a rotary blade fan and you can move a good quantity of air with a relatively small fan. The fan's CFM shoud guide you as to size.
Slatted wood shelving - perhaps maple for its resonance-draining properties - would facilitate air movement although these may need to be custom made.

For fans look at W. W Granger catalog, they have ones for every possible use. If it were me I would use something like the Star Sound rack that I use. It uses mainly open steel shelves with the components mounted on cones coupled by discs. Gives the best isolation I have has by far and you can build an enclosure with doors to hold it. There are other good racks available. I think this would give you better sound and the cost would probably be comparable to a custom built cabinet. With any custom cabinet , no matter the care taken, you never know how it will sound. Years ago Martin Colloms had a custom built cabinet made, even placing it on a large marble slab to add mass and so it could slide on carpet. It weighed several hundred pounds but was not a success despite his best efforts. If you use a rack you will at least know what you are going to get and many of them are very attractive.

jgialco is SOOOO right. Listen to him totally!