I will say, if this is for home theater, then where did you plan for the center channel to go? It may not matter for your setup, but I wanted to ask.
I would recommend a depth of 26" from the front rack rail to the back of the rack (inside dimention) and you need about 1" or so for any products with buttons/knobs to stick out.
Ventilation should be from the bottom up, with a sealed environment except for a clear path of intake at the bottom and a proper output location at the top/back of the unit. I can tell you that in a dozen years, I've seen almost ZERO professional woodworkers who actually know how to build A/V racks, even though they do know woodworking very well.
You want to consider any plugs or outlets which are behind the credenza as well, as they will have some wall clearance you have to deal with. You may want the back of the credenza to stick out 3" so that you can get the credenza closer to the wall at the surface, but leave room behind the rack for external cabling along the wall area.
Rear rack rail for cable management is a huge plus.
Square hole or round hole doesn't really matter. The square holes are nice because you can switch out the cage nuts. But, the round holes are easy to deal with and just require a bit more care on initial installation.
Since it likely IS going against a wall, I would go with a caster base, not feet on the unit. This isn't to store books, so you need REAR access to the credenza. The feet go away and get replaced with casters. A skirted base to hide the casters is an option.
The rear access panel should either be 3 doors, or preferably one single large panel which can be removed to give full access to all three bays from the back side easily.
There should be a cable path from left to right which is a notch to hold the wiring in place, and have plenty of room for that cabling. It should not just be an open pathway, as that leaves some wiring headaches from behind. A C-channel at the top and the bottom of the rack to allow wiring to be put in place is best for wire management, looks, and ventilation.
If no gear is going in the center section, just storage, then make sure that center section is built to hold what you want to put into it.
Make sure the rack rail has about a .5" offset from the edge, perhaps more on the side which has hinges so that equipment can be installed/removed from the rack while still on a proper rack shelf.
Feel free to PM me or respond if you have more questions or need more details.
The ONLY part of this which adds money is proper integrated fans. I saw a website at some point which has cheap, quiet, thermostatically controlled fans, which would be worth putting in yourself if the woodworker charges to much (possible).
