So was going to drop an update here to say I've got my new (old) system up and running, but was cleaning up the parts this weekend for assembly and had a bad surprise when I went to install the CPU. A bunch of pins in the socket seem to have been torn out. Nothing to be done but contact the eBay seller. Unfortunately I was delayed getting round to this and I'm probably past the returns period.
On a related note, my plans to install a heatsink would have hit a snag anyway. There's a backplate glued in place (marked BKT-0028L) that would block the installation of a stock Intel heatsink. How would one normally deal with this? Can it be removed, or is there a particular special heatsink I could source to fit?