I've seen a few of these questions come and go over the years, and they have a disturbing tendency to get closed as NARQ -- Too Broad. Which is sad, since there is a nice corpus of knowledge to tap out there.
A 5-rack datacenter is a different animal than a 2500 sq/ft facility, and the challenges are correspondingly harder, so we definitely have a diversity of environment to work with. The trick is scoping the questions narrow enough they'll get answered.
I think they're topical, honestly.
To elaborate a bit more, I think there is definitely a cut-off point when the sysadmin should be involved and when hire a professional kicks in. When designing such a facility, be it a 2 rack closet or a 5MW facility, you need at least one person familiar with correct datacenter design involved with the project. Dedicated datacenter architects do exist and you can even hire them, but that doesn't mean they'll be contacted by whomever you're working for.
All too often, especially for smaller projects, it's the building facilities people who are extremely well versed in keeping infrequently occupying moist hairless apes happy who will be given the task of engineering the exact airflow details. Keeping steady resistive loads happy is a different problem all together, though related, and the assumptions for one don't always carry to the other. I've been told by more than one such professional:
Oh, your servers aren't used as much at night and weekends, so they'll need less A/C.
Um, no. No they aren't. We run those babies at full tilt 24/7. This is a mythology that seems common in office-building engineers: IT equipment directly supports the people in the building, and gets turned off or goes low-power-mode when the people leave.
Speaking knowledgeably about airflow and power allows you to be more effective in urging the worst of the bad ideas to not make it into the final build. Ideally, you'd have someone actually experienced in that kind of engineering, but in lieu of that you can be the shoulder devil for the trained but inexperienced person who is having to draw the diagrams and order the parts.
ServerFault can act as that shoulder-devil if you let us. We have enough of the right kind of people to poke holes in plans and sound like we know what we're doing while doing so.