don’t get worried about the engine. A SWB car (all SI 2.8l are SWB) with manual transmission (is it with overdrive?) is an agile beast. The small block wants to rev. The poor reputation of these engines stems from burnt pistons caused by carbon build up. It can be avoided by regular committed driving.
Be aware that even the smal block is a big chunk of iron with a lot of water and oil in it. While water temp will climb within minutes, oil temp takes like 20 minutes - not idling or (sub-)urban traffic, but serious driving. So your 20 km stretch would almost certainly be less than ideal for the car.
As for winter use: there used to be block heating units available and they were widespread in Scandinavia. Either your car has one or you can get your hands on an original one. There may even be modern sources of such parts. I’d get one for sure.
Heating is not as vigourous as the amount of fuel used up by these cars would have you believe. In particular roof insulation is critical. If the original foam headliner has been tossed out and the headlining material has been glued directly to the metal you’ll lose lots of heat through the roof.
Driving on snow with the manual transmission RWD is no more problematic than on your C class. Put some sand bags in the trunk if you feel like it or use studded tires if you’re allowed to do so.
Rust is a big issue on these cars as the body structure is complex and repair in critical areas difficult and expensive. Deep cold and no salt is uncritical though if you rust proof the car - either waxoyl or grease (Mike Sanders etc). Many of the best original cars from the 1950s to 1970s on the German market are imported now from northern Scandinavia. Slush and salt is hell though. You won’t be able to protect your car against it - just avoid exposure.
Other than that: drive that car as spirited as your local regulations permit. It is a four-door grand tourer. Many systems on the car follow the rule “use it or lose it”, e.g. electical components such as power windows, switches in general, horns, but also brakes.
75 XJ6L 4.2 auto (UK spec)