5k is a smart upgrade for a s85D vs s85 for a non Snow belt climate and why?

I would also guess the D would have higher repair cost due to the additional moving parts? And smaller frunk.

The positive I can think of is perhaps splitting the work would put less strain on a single component.

It's a personal choice with your level of comfort. I have an S85 and went skiing 10 straight weekends including some significant snowstorms and never had any concern about the weather. I do have dedicated snow tires. Having said that, I never had to drive through more than about 3" of unpacked snow on the roadway with plowing, etc. There are advantages to AWD beyond performance in snow, but I would not buy it just for the snow performance.