I think I'm just thinking "out loud" with this post, not seeking the answer to a question. That's why it's in non-NIX general.

My preference for buying, building and using 10-15 year old technology started because of a mistake made in, I think, 2013. I thought I needed a laptop for a certain project that forced me to use some Windows software, so I ran to eBay and bought a used laptop with Windows installed. I simply didn't think of buying a copy of Windows, installing it on a separate partition, and running dual-boot. Of course I knew I could do that, but didn't think of it.

The first laptop I found acceptable for my purposes and desired price was apparently a ThinkPad T61. I became so comfortable with it that I spent the next ten years using mostly these used Thinkpads. (They all die eventually, necessitating replacement. It's only a question of how long each lasts.) I like ThinkPad T series laptops because they're designed with modification in mind. It well withstands being opened up and poked around in. (The HP laptop I have right now obviously was not built to handle being opened up. The frame is already damaged from being opened even once.) And you can't even change the hard drive on an HP without opening it up. Whereas I've never seen a ThinkPad T series laptop that didn't have a hard drive bay accessible from outside.)

In early 2023, I simply got tired of buying a used laptop every year (if lucky!) or so, and decided to return to building my own desktop computer. But my habit had taught me that I clearly didn't need the latest greatest technology. My needs were too limited. So I decided to deliberately build it from working but obsolete parts. The CPU and motherboard are both approximately 15 years old, roughly the age of the components in most of my ThinkPads. The CPU was used and the motherboard was old but supposedly unused. My desktop computer build works, but...not perfectly. I wonder if my seller lied about it being unused. Sometimes I think it would be simpler to just go back to ThinkPads.

I just know I don't want to build a desktop computer with a new motherboard and CPU. Waste of money: I'd probably never take full advantage of a modern computer's capabilities.

I've come to a different place for a different reasons. I'm an ex-hardware guy.

Things are always becoming obsolete on PCs. So go with current hardware, because that will be obsolete by EOL. Don't buy laptops unless you need one. A desktop is like Lego. If something breaks you can replace it because there's competition. Not so necessarily with Laptops.

My last box was from a pc builder, because I can no longer build, with one arm paralysed etc. I learned

• From my last Laptop, the lesson was get a separate video card because ALL IGPs suck, and all want to take over.
• Many CPUs come in two versions. Mine is a Ryzen 5600. There is a faster 5600X. I since discovered that if a die fails as a 5600X, but passes as a 5600, it's sold as a 5600. If you don't want rejects, get the better CPU.
• Accept that as soon as you put your money down, what you bought will go out of date.
• If buying from a PC builder, never get their least expensive option, because it's always a cheap POS.

I don't know if they still make them, I used to buy slim clients and made them Linux.

Actually a Pi is just as good. I have screens, keyboards, mice, and either the SBC has WiFi or I can get a good USB WiFi adapter. Only other thing is storage. I have a USB hard drive and can buy a SD or uSD for the SBC at time of purchase.

For my Linux desktops I've always bought used Dell units, and I've had good luck with them. Up until recently I bought refurbished laptops as well. However, this time when I needed a laptop I bought new. I got a T14 Thinkpad which was marked down a lot. I ran Win 11 Pro for a while and a few distros (Debian and Void) and settled on Ubuntu. The laptop runs well. My desktops run Slackware.

I've been using pre used computers for over a decade, like you, I don't need the latest, just a desktop, as small as possible, to go online with a web browser at a reasonable speed, play some time waster games, music, & videos.
(Other than that, I only use a spreadsheet to keep track of my money.)

(See my signature below.)

Actually, I once needed to do just what your title says. I got 7-10 year old PCs to repair. Due to devices being made obsolete, they needed upgrades. Here's what I did

My business took me around town. I would stop off at smaller PC shops, and say:

"Hello, do you do upgrades?"
This would start off a sales spiel, which I would cut off.
Good. Do you have any DOWNGRADES?

I would then explain that I would pay them beer money in cash for any working motherboards, PSUs, Hard disks or whatever I needed right then. This would go in their pocket. They would make clear there was no guarantee, which I would accept, and honour. I would buy what worked for cheap, and put a good mark-up on. I never once got let down by these 'downgrades.'

Nice! As I mentioned previously I would dumpster dive on ebay and get laptops that were marked in at least "good" condition. I ended up returning 3 laptops that would either not POST or the seller "embellished" the specs. Hence my decision to buy new this time.
I have two working desktops down in my basement(technology graveyard) that I can swap in should one of my Slackware towers fail.

I bought a ThinkCentre for $40 on eBay. It didn't work. I bought another from a different vendor that also didn't work. (I put many days into trying.) Shipping back would have cost more for them so they just let me keep them. I cobbled together 1 working computer from the 2. If I had billed myself minimum wage for all the time I spent it wouldn't have been a good deal.

My previous computer is a Toshiba Satellite laptop. The hinges broke, as did the audio jack, but it still works otherwise.

What you'll find is that for the financial cost of doing that, you can buy yourself something new and bottom end.

When many of us were young, stuff was expensive and labour was cheap. Now labour is expensive and much stuff is cheap. What do you value your labour at? Some might say "$0 per hour, because it's a hobby and I have oodles of time on my hands."

If you're not in that group, surely some practical thinking ought to enter the equation. If you spend a lot of time assembling cast-offs you are going to get an unreliable working castoff. If the cost is parts only, it might be cheap. OTOH, if you price parts & labour, it might be expensive.

There's a local very-reputable computer shop that does a back-room business of selling, or even giving away, "old" computers. (The rapacious appetite of Microsoft Windows to these days run only on the "latest" hardware has driven a lot of disposal.) They will even install Linux on it for you (since Microsoft "frowns upon" installing "older" versions of Windows), and shop-warranty(!) it for one year. If the residents of a local retirement home simply want to "get on the internet," the shop will equip them for free. (Cleverly attaching a sticker to it with the name and phone number of their business. "The kids" notice such things ...)

If "older hardware" will do the job for you – as it almost always will – you can get stuff practically free.

These days, there's a lot of "manufactured obsolescence" in the computer game. Because computer hardware can otherwise "run quite satisfactorily" for a long time.