Hi,
I am a local to the central Oregon area as well (for about 2.5 years). I would just like to throw out there that if you don't want to buy a second car, you still have options. First, have you considered renting a car from a mechanic? They often have longer term rentals available, are more likely to negotiate and probably wont mind if you want to put on snow tires. If you spend $1000 for a one month car rental you will still probably come out ahead with not buying. You could also consider posting on facebook/craigslist that you need a vehicle that fits parameters X, Y, and Z, for a month and would be willing to pay. There are many kind people in central Oregon that would happily give up there second vehicle for a month or two while there college/high school kid is home in exchange for some Christmas money. Check with your local friends as well, you never know what they will have.
If buying makes sense for other reasons, I have found the best place to look for a used car is facebook not craigslist, as the prices tend to be more affordable. My current 2002 Toyota Camry has 275,000 miles on it, and my mothers 2001 Toyota Avalon has 250,000 miles, both have not had major maintenance expenses, although the Camry is nearing the end of its life. At this point I assume any Toyota or Honda will last to at least 250,000 miles with minimal problems. Subaru's are pretty popular around here, but I don't have any experience with them, most have all wheel drive which can be nice, though some years have pretty bad head gasket problems. Do your research before you buy.
When I am looking to buy, I try to do a cost per mile calculation. If I assume that Honda's and Toyota's will last to 250K miles, and all other cars will last to 200k miles, what does the price per mile calculate out to for different cars of different miles and price points. I try to take into account fuel is $3/gallon, and assume insurance prices and regular maintenance will be the same. This is the formula I use, but feel free to make up your own.
Keep in mind that if you are getting a second car, you are unlike to put as many miles on it, as I assume you will not increase you driving more than 10% and the other car will still get driven in the snow or to the mountains.