I grew up with Apple IIe/IIgs and Apple Macintoshes in the K-12 school system (although the high school finally updated their technology and got all new Windows 98 PCs), but I'm really a beginner and never used macOS X or anything more modern, so I'd have to start from scratch.

That's great there's an Apple Refurbished storefront, especially if it's an official Apple store. I've bought a lot of my PCs from Dell Refurbished (with great 40-50% off coupons or their clearance "hot deals" section).

For crunchboxes running BOINC and other distributed computing projects, I'm still really all in on Linux since it's free and can be run headless without a silly GUI.

I'm still a Windows 10 Pro user for my daily driver, but I've been putting off really switching completely to Linux for many, many years, since I'm so ingrained.

That being said, if I need a non-Windows, non-Linux device, I'll definitely look into Apple Refurbished! Maybe to use DaVinci Resolve or something, not sure. I'm really philosophically trying to get completely away from Microsoft as well as Apple ecosystems. That's just my personal goal, but I do respect the Apple users out there and thanks for sharing those prices. $300s is WAY cheaper than a new Mac Mini or Mac Studio.

I've some experience with the Dell, Lenovo, and Hp mini systems, especially around the Skylake era.

Overall they work pretty well, and I've not had any failures in a couple years of crunching, though some of the systems are still relatively recent additions. They're available extremely cheaply, especially if you're willing to do some assembly yourself (disks and power adapters not always included). My record thus far is about $40, though average is probably closer to 60-80.

For noise, Lenovo is the winner hands down, no contest. Hp is in the middle. I have one with a Pentium g6400t that ties the Lenovo team for barely-audible behavior and suspect newer Hp builds may be quieter, but in general Hp is noticeable. Dell... is very loud. If you care about noise levels go Lenovo. You won't notice them unless they're right next to you on a desk, it's 80 degrees (~25 C) in the room, or you're running half a dozen in the same cubic foot.

My machines return roughly 60 or so MCM results a day, with slight variations depending on the chip. They're all 35w or less.

You can get stronger builds if you want to pay more, but my goal was compute for cheap, and these sure beat out Raspberry Pis in terms of processing strength.