What's not clear to me is if I update a modern system to Win 11 with TPM installed, does MS always to try to upgarde it again and again after Win 10 is restored and TPM is removed?
It's my experience that Microsoft will continue to offer the upgrade every few weeks even on hardware that is compatible but using legacy boot. Legacy boot is, by definition, NOT compatible.
I'm not understanding who wants to put 11 on an old computer, some kind of masochists?
I have a hard time calling a 7th gen Intel PC "old" (four years at the time Windows 11 was released), especially when Microsoft had to carve out an exception for one of the Surface Pro models
it was still selling when Windows 11 was released. Windows hardware requirements have barely changed since
Windows XP was released: 1GHz CPU, 512MB RAM (actually, it'll install on 64MB if you tell it to), SVGA graphics and 25GB for 32 bit or 50GB disk for 64-bit architecture. Windows 8.1 added a requirement for NX bit support in the CPU and BIOS, which was mostly a problem for Pentium 4s and AthlonXP machines, and 11 theoretically requires UEFI. There are very valid reasons to reject UEFI boot, starting with the fact that Microsoft is the sole entity that can sign a boot loader. That's a problem for some open source purists, but it also breaks A LOT of useful tech tools.
IMO, if it uses DDR4, it's probably going to be subjectively fast enough enough for most daily use purposes. I'm spoiled and generally don't use anything old myself, but some of the offices I support are still using Haswell-based PCs and I defy you to tell me they aren't fast enough, especially for i5/i7 grade CPUs vs. contemporary Alder Lake dual core DDR5-capable Celerons.
One of my concerns would be the drives failing to work when you need them. Anything second hand could be heavily used. Also, with drives being mechanical devices if they sit unused for a while they might not work. For safety reasons, I'd probably want 2 or 3 drives on hand just in case of failures.
As I already said, there are an oversupply of drives and they were made to be heavily used. I've never personally had one wear out, albeit that's from a sample size of maybe three dozen units I've been directly responsible for. Helical tape systems, the parts that are likely to wear out, are something that are pretty well understood. We've gotten pretty good at making them work over the last sixty or seventy years.