4-6 months ago I built a new workstation for work. I had used one of those Corsair closed loop water coolers with the prior build, so I set one up on this guy. A month or so later my workstation was running really sluggishly, and I realized it was drastically throttling the CPU because of heat. I installed some software to spin up the fans on the cooler to keep it down under 100C, but now it'll get kind of loud when I run much heavy CPU.
Now, this is a pretty heavy duty workstation, 64GB of RAM and 3 displays. But, if I were doing a new machine for home to be quiet, I think it'd be a NUC. Then I'll put the box that has the 6 drive ZFS array in a closet and call it good.
Their highest end is available with ECC ram and a discrete graphics card: https://fit-iot.com/web/product/airtop2-build-to-order/
And the lowest end fitlet2 can be configured with an atom in a reasonable configuration for around $300 (it's $130-$200 for case/cpu/motherboard depending on which CPU you configure).
Gigabyte also has their BRIX products, which are similar to the NUC.
I was at a hotel this weekend and at check-in they had a monitor with a "ThinkVantage slotted in the back of the monitor, that might be a nice setup.
Eventually I just bought a cheap USB desktop fan and ran it facing the NUC.
It has made me think that instead of getting a NUC, for a quiet desktop system I should have just gotten a second used MBP and ran it permanently docked in clamshell mode with the monitors and keyboard attached. (with the added benefit of being able to go portable when I want to)
Neither the NUC nor my MBP is completely silent but for my purposes I find that I seldom tax either of them enough to where the fans become audible enough to be annoying. Still, I do find the difference in performance between them to be apparent just in things like iteration time on web development and IDE responsiveness.