It cannot process unicode characters so stick with the NT map version ("non unicode") as said by Garmin_Nuvi or you may get errors during use.

It might even still be up-to-date but if not you can get the "latest" firmware version 6.10, issued almost 16 years ago, from here:
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or you can just connect it to WebUpdater to get latest version as well as any available updates for Voice etc.. If you need WebUpdater you can get it here:
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Because your SP c320 doesn't have Mass Storage Mode it won't be supported by GarminExpress which is a much more modern software than WebUpdater. However WU is perfectly functional for updating such old 'region-only' devices which are without the function to browse the internal memory.

By "base image files" if you mean the basemap and time-zone map, it doesn't use the latter and the former is contained in region 3 of the internal memory which is likely quite size-restricted so a modern World Wide DEM Base Map would almost certainly not fit in region3 despite it being only ~46 MiB. The gmapbmap.img on-board would be a quite old EU non-DEM only (maybe it's even just a partial, like eastern or western Europe, idk) however despite the EU naming it would contain some basic information for the rest of the world so even if it's only East EU you can still use it elsewhere. A Basemap is generally only fully utilized if a navigation task goes beyond the coverage of the detail map or when manually scrolling the map while zoomed out, but of course must be present for glitch-free navigation. It is the presence of it, not the age of the basemap that's of great importance. In fact Garmin doesn't even bother offering devices any basemap updates via WU or GE for that reason.

I don't recall if that series can use Safety Cams alerts but as i doubt gramps will be running reds or speeding going by your description of him, you can skip them anyway. If you want me to make you a custom Logo splash screen to flash to Region 16 reading something like "Welcome Captain Slow. Don't forget the go pedal is the little one on the right!" - Just give me the wording in the language you want.... nothing naughty though, we don't want the old bloke to get a nasty shock first time he boots it up do we?