When buying a second hand system for Windows 11, check if the CPU is on one of these lists for Intel and AMD processors:
https://learn.microsoft.com/en-gb/w...ed/windows-11-24h2-supported-intel-processors
https://learn.microsoft.com/en-gb/w...rted/windows-11-24h2-supported-amd-processors
As a rough guide, most processors released before 2018 are not officially supported by Window 11. I can see quite a few 8th Generation Intel CPUs on the list, plus 9th Gen. to 14th Gen.
I doubt you'll be able to afford most of the Intel's 14th Gen. CPUs, so look for something in 9th to 11th Gens if you're still considering the second hand market and $300. Ditto similarly specified AMD CPUs. I have an equal mix of Intel and AMD.
When it comes to processors, the more cores the merrier. A 2-core CPU will be horribly sluggish. 4-core (preferably with 8-thread) slightly better. Next there are 6-core 12-thread and 8-core 16-thread, but more expensive. Recent Intel CPUs have abandoned the concept of "threads" and concentrated purely on "cores".
https://techcult.com/cpu-cores-vs-threads-explained/
In terms of CPU speed, I'd look for something with a base clock rate in the 3.4 to 4.0GHz range. Anything slower than 3.0GHz will probably get bogged down. Anything faster than 4.0GHz will likely cost more than your $300 budget.
When it comes to system RAM, Microsoft mention 4GB as the minimum for Windows 11, but I regard 8GB as the absolute lower limit. 16GB is better and should suffice for office work/web browsing. Some programs work better with 32GB+.
It's often possible to add more RAM to a second hand machine, if you find an absolute bargain otherwise, but with too little memory.
Assuming the new PC has a spare interface for your old Windows 10 SSD (i.e. a SATA port + power lead, or an empty M.2 SATA/NVME slot) you can transfer the old drive
for use as a data repository.
If you want to boot Windows 10 from the old drive in the new machine, you may encounter problems. Booting Windows on a different motherboard might work fine, or end in disaster and corrupt some of your precious data files. Backup anything vital before trying.
It's not "good" practice to move Windows drives between machines and boot from them, but I haven't encountered many problems on lesser systems of no great importance. Windows usually detects the new hardware and installs many/most/all the appropriate drivers.
Best practice is
always to perform a clean install of Windows, to avoid unnecessary problems. I can install Windows on SSD in less than 20 minutes, plus 2 to 3 hours to tweak settings and install additional programs.
My advice is to leave the Windows 10 drive where it is and use the old machine as a backup, in case the new Windows 11 machine ever goes wrong.