This will upset some of you!
I would recommend eBay and look for a pre 2011 27" iMac (collection only as they are heavy and fragile). I say pre 2011, because after that date screens were glued on replacing the easy-to-remove magnetic ones.
Now you can get inside (plenty of YouTube videos) and fit a SSD (240 GB is fine for size for me). Upgrading the RAM is also easy. Off you now go with Linux or dual boot with a Mac OS.
A lovely, lively desktop machine!