If it's general, everyday use and not a specialized purchase (i.e. getting a Raspberry Pi to set up a Pihole) then you will not get a good computer for that cheap. When I say "good computer" I mean something that will meet your expressed needs and give you a smooth experience, without anything fancy that you don't need.
One can save money on refurbished parts, but I don't trust them because they failed on at least one person. Maybe they're fixed, maybe they're not, and if they're not, you have to spend money (again) to replace them anyway.
Used disks - forget it, you don't know what's been on there, and data is actually very hard to destroy to the point that governments or certain thieves won't be able to find it. You don't want to inherit a drive that may have had shady stuff, or even just a really bad virus on it. I've had to delete malware and then hit the free space with a Guttman wipe to stop it from coming back immediately, and that old computer never really ceased to have infection problems.
One can get older, but still solid computer parts - motherboard, CPU, RAM, case, fans - and combine them with a fresh power supply and a fresh SSD, and put a lightweight Linux distribution on it - if one is technical enough and wants to learn the intricacies of such a project. Most people I know of that qualify won't be looking for 9+ year old HP desktops on Amazon, not unless they need it for specific parts to fix another one.
One can get a super-basic, new desktop without a monitor for as little as $350-400. You might want to add a little more for an extended warranty, plus obviously the cost of a monitor if you don't have one. Figure tax, shipping, you might want to budget $500 or thereabouts for the purchase. This is just for a boring old office machine. I don't pay for MS Office or Outlook, I use LibreOffice and Thunderbird for free. I use the free MalwareBytes and Avast Antivirus because I'm confident and careful. I have recommended and implemented paid ESET NOD32 and the premium MalwareBytes on machines that are for less technical people.
I'd say save up the proper amount of money and buy when there's a big sale.