The iMac booted to Mint after the install successfully, however, as before the wireless drivers were missing, so for that solution, see the spoiler above. The only question now is if the install/boot ...
Long story short - I've got my Mum's old iMac which is a Core i5 2500S vintage machine. 12GB RAM, 1TB spinning drive. Still quite a nice unit, screen is still good, so trying to see if we can make use ...