IIRC, these top out at High Sierra and cannot run Mohave?
Yep, HIgh Sierra will be as far as these can go.
Is it worth buying even though it’s refurbished?
If you get this, I’d recommend getting an extra stick of RAM and doing an SSD upgrade; the HDD will feel very slow. The kit here (OWC.com) comes with suction cups to remove the screen for replacing the innards. I did this to my 2010 iMac a couple years ago and it’s still going strong (except for one stuck cyan pixel ).