Do you want the printer to be a tool, or a hobby (i.e. you don’t mind fiddling with the printer itself to improve the results, you don’t mind spending more to upgrade components, etc)?
If the printer itself is a hobby you can go cheap, but if you want something reliable you don’t have to mess with or upgrade, I’d suggest getting something as nice as you can afford, maybe a Prusa mini or Bambu A1 mini if you don’t care about open source. Also consider something like a used Prusa Mk3.x.
I haven’t bought used myself, but based on my own experiences with my printer (MK4; I love it) I don’t think there’s a ton to worry about buying a used quality printer. I would not buy a used low-end printer because the odds are much higher that the seller found it frustrating.
A used Prusa MK3S is probably an excellent choice if a Prusa MK4 kit is out of your budget and a Bambu printer is out of your budget or doesn’t meet your other requirements. The seller probably either realized they don’t actually print often, or upgraded to a MK4 (or XL if they had the budget). While you can upgrade a MK3S to MK4 with Prusa’s upgrade kit, the cost of the kit is so close to just buying an MK4 that it’s not worth doing (and Prusa admits this; they only offer it because of the flak they got for not doing such a kit in the past). The MK3.5 or 3.9 upgrade kits could make sense for some people…but in many cases someone looking to upgrade would likely leave the MK3S untouched and just buy an MK4.