That would make around 25€ .for £20 each...
Such 2nd hand Optiplex are sold around 130-250 € here in shops. The price mostly depends on RAM size.
My are fair solid.
HDDs were running for another 12 month with much rsync, mksquashfs and make.
Have replaced/upgraded them after that time just in case.
Serious problem had with untight speaker and fan cable joint ends to the board ( no sound or fan sometimes did not run ) .
They have memory check in the BIOS.
As ally says : Check the memory bar(s) .
The smaller Optiplex seems to need pairs of memory bars.