The BMW should be fine in the snow - we have a RWD 3 series and even with just all-season tires it handled the April snows just fine. We took it to Keystone over Christmas without issue as well. We had an xDrive loaner recently (that's 99% of what the dealers stock around here, it seems) and the standard non-sporty suspension was atrocious. Leaned in the corners like a Camry. Stick with what you got, IMO.
With respect, you've just been lucky. You NEED snow tires on that vehicle if it's your ski car. (Yeah, back to tires!!!) I drove a RWD sports car all through college (100's of times Boulder-->Mary Jane) as my ski car and it can be done, but only with four snow tires. I did it with with four studs; you might get by with studless, but I'd avoid high performance studless for a ski car. HP studless are OK for getting around the front range, but not the mountains.
Another option is my plan: I've got a RWD G35 that I leave all season's on year round. It sits in the garage when it snows. Even then, I don't run HP tires because they are outright dangerous in the snow, even on the front range. I keep an extra vehicle around as my ski truck: an older Tundra with four studded snows. I can confidently drive the "Tundra Express" into any storm, and I pack a tow rope to pull Suburu's and 4WD's without snow tires out of the ditch.
We dont want to live IN Longmont, but somewhere around Longmont. We're just going to drive up to Denver in the next week or so and start looking up property rentals. Everything we see listed is in town, we would prefer in the country. Like you said, with a shop, or at least enough land to park my race rig.
I think it's going to be really tough to find a rental that meets your need, as you are finding. Here's one idea: properties like you want probably take longer to sell. Maybe you can find one that is for sale and vacant and work a deal where you lease it with an option to buy when your home sells. Just a thought.