I would ask, why you want an older machine regardless of the cost. Before I did that, I would want to have a specific purpose for the machine versus buying it and then figuring it out.
Something I would consider, going from Win 7 to Win 10 or Win 11 which is now current is likely a rebuild versus an upgrade.
I would do a little googling and see what the minimum requirements are for Win 10 or 11, depending on what you decide.
Then I would see if the machine you are working has the memory and processor capable or running either OS. Memory is normally cheap and is the easiest to install and get immediate results.
Then I would serious consider what it would cost me to make the machine you are working on run either Win 10 or 11 and is it even worth it, compared to the cost of a new more modern machine.
Huh? This is the newest and fastest desktop I have, with a SATA 240 GB SSD and 8 GB of DDR4 RAM (I’ll upgrade to 16 or 24 GB.*) Only a 6th gen i-7 processor, so “by the book” Microsoft doesn’t guarantee Win 11 on it, but, its running Win 10 Pro quite nicely and fairly quickly. I’m in the process of upgrading to a 500 GB M.2 NVME UEFI boot drive (genesis of previous thread, as said upgrade is a bit more complicated than expected, then the SATA 240 GB SSD will become a data drive for now - later I’ll add a larger SATA data drive and the 240 GB drive will become a (primarily) downloads drive.
ALSO, this machine has both HDMI and (2) DP ports, and lots of USB 3 ports — perfect for me.
*Actually, the RAM upgrade would already be done but it appears I got a bad “stick”. (It’s exactly per Crucial’s scan / recommendation.)
No, this isn’t the latest “screamer” / gaming monster, but it’s more than sufficient to be my primary office machine for a year or two, at which point I can likely nab a Win 11 Pro or at least a Win 11 Pro capable machine for under $100.