i would highly recommend 2019 iMac hardware on a budget
2019 runs Mojave 10.14.6 to Sequoia 15.7, and there are plenty of used parts -- try finding a used screen for silicon iMac and you will pay an arm and a leg for it
I would avoid anything with HDD (spinning hard drive) -- remember an Apple fusion configuration is actually 3 percent tiny fast SSD and 97 percent slow HDD
I would also avoid any machine with minimum RAM (8gb) -- 16gb or better, 32 minimum if you run photoshop or video apps
NEVER BUY A USED COMPUTER?
I can only comment for Apple computers, iMac
if you are the type of person that never buys a car (except from a branded dealer) -- then buy a new computer
BUYING A USED iMAC is a lot like buying a used car -- except a lot less can go wrong
generally all I do is look at a DriveDX report for Drive Health and Power On Hours
I look for at least 98% health and under 5000 power-on hours, no fail flags
if that checks out
and it looks good overall
it come down to the seller, their story, would you buy a used car from them?
i've bought more than a dozen used 2019 iMacs off local ads, including eBay, and never had issues with them following drive health, hours, seller vibe -- i've walked away from a couple deals based on the latter