[jdcarlson]

It all depends on what equipment you will have.
If you are looking for space for higher end (i.e., industrial strength) double conversion power supplies, you will need at least 26".
Also - although they have fallen out of favor, the 400 CD "jukeboxes" were too deep for a lot of earlier racks & cabinets (I found out the hard way after I had a custom built solid mahogany credenza that was only 22" deep).
Even if you don't think you will need the depth now - plan for the future.
Incidentally - 19" professional racks come in depths up to 30". Google Markertek. They have several choices of industrial racks. Another source for industrial strength racks is MilesTek (similar name to the other - but a totally different approach to marketing).
Just a warning - I am talking about racks pre-drilled for mounting 19" industrial equipment. That is what is used to make the recordings we are spending all of our money to listen to. But, some of the racks have wood-tone sides & tops, with smoked glass fronts. They can (if properly chosen) look good enough to pass the wife approval factor (WAF) test.
Thanks,
Jim