I guess I've been doing something similar. I currently use 9 laptops (2 Windows, 5 Linux, 1 MacBookAir, 1 Chromebook). Only the MBA11 and the Chromebook were purchased new. The rest were bought on eBay for $250-$600. After getting them, I max out the RAM, replace the HDD with SSD, or upgrade the SSD. A few of them needed some parts replaced (e.g. fans). The battery almost always needs replacing at some point. Fortunately, that's easy with these older laptops. Even the MBA11 has a replaceable battery because it's from 2013. Out of the ~15 laptops that I have purchased on eBay over the past ~15 years, I think only 2 have had serious hardware issues.

I seem to have standardized on Dell laptops, mostly because I have drawers full of their interchangeable accessories, like power adapters and docking stations. I've been happy with their Precision line, or the higher-end models of their Latitude line. [Edited: to make the relationship between Precision and Latitude more clear.] The sweet spot in age seems to be 2-4 years old, probably when corporate users are refreshed, and the old ones are wiped and sold.

Off-topic: Does anyone else have a hard time reading the article due to its blue/orange background? I find it visually painful to read a paragraph when it is split into 2 colors. I have to scroll the paragraph to one color or the other.