While they pontificate about proper tire diameter, I would suggest two other factors to consider. Tire age and tread depth. Learn to locate and read the 4 digit DOT date code on tires (if it's 3 digits, the tire is from 1999 or older). It could be particularly useful when buying used snow tires because these tires may have been sitting around for 5yr ever since they sold their 240, and now the garage is being cleaned out.

And ask for tread depth measurements, or measure them yourself if/when you pick them up. Most people who quote tread by % remaining just don't know. Miles driven is also not a great indicator of treadwear. And the rest show depth on a coin. At 10/32nd, they'll probably churn through anything. At 5/32nd, still decent depth, but they will not be nearly as impressive.

185/75/14 is more than adequate for a 240 wagon. Search google for a picture of each tire to get an idea of the tread pattern. Some snow tires look like normal all seasons while others have small, chunky tread blocks with wide spacing. The latter will often be great in deep snow, but may give the squirelly ride and sloppy handling that snow tires are known for.
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