I still think the first one I posted for you the All in One is the best price for everything.
You could buy a used computer but there is no guarantee on them. If you have like a pawn shop or a used computer shop around you then you can find one through them.
Please understand that a new system is going to cost to get regardless of building it yourself or buying a name brand system.
Since you can't update that one then your options are to either buy a used and maybe inherit problems or buy a new one and have a warranty that will protect you for a year at least.