I'm looking at buying a used PS4 for playing offline games from disc. The best deals by far are on systems that have been banned from playing online so I was wondering if that would affect me? The only thing that comes to mind is system updates - can a banned PS4 still download them?
Yes, you can still download system updates for a banned PS4
You can see what's new in the "What's New" section, browse the online manual, use the Internet browser, download new system updates, and use Remote Play using wireless LAN.
(emphasis my own)