Hi Geoff!
I have owned a 2014 Smart Electric Drive, which I purchased used, for the past 3 years. I can say that it's been a trooper and a tank. It has a liquid cooled and heated battery that has been very robust so far. I haven't noticed any degradation after more than 70,000 kms.
My comments will be a bit generalized, since you're looking at the newer generation than mine (the newer 4th gen EQ vs. my 3rd gen ED), but I'm pretty sure the drivetrain didn't change between my model and the one's you're looking at, or at least didn't change significantly. Having said that, I don't think there's anything specific to the EV drivetrain to look out for on these cars. They're pretty solid. You mentioned that you've owned a diesel one before, so you have some experience with the car as a whole.
My experience in the winter is that you'll get a reliable 50-70 kms out of it, depending on whether you're doing a lot of short trips or one trip out and back. Multiple trips kill the range because you're repeatedly heating the car up. If you're just doing one trip out, and one back, I'd say 70 kms is probably a pretty reasonable number. I used to commute about 70 kms round trip and did that through winter. I was arriving home pretty much on empty though every day. Trying for 100 kms in the winter is pushing it a bit too far without having some charging available at your destination. Getting 100 kms range is pretty easy in the warmer months if you drive easy.
Other than the loss of range, mine has been fine in the winter. Since you've owned a Smart before, I assume you're familiar with the winter driving dynamics. It's a short wheelbase, so it does feel like it's moving around a bit more on slippery roads, but the traction and stability controls are top notch. It's handled Sudbury winters remarkably well. Snow tires are a must. Otherwise, it's been great.
The preheat function on my generation of Smart ED is notoriously crappy. It is a pain to set up and doesn't really work. Look into the 4th gen EQ to see if the preheat function is better, you might be able to cruise some of the forums for answers there. I don't know if it has an app.
Best of luck!