You will get a reasonable impression of the availability of Bank of Montreal notes in the September issue of the CPMS Newsletter - it's not out yet, but I've had a sneak peek (heh heh). It should be available by mid-September, I'd say. The Bank of Canada $25 1935 is not a rare note at all, it is merely very popular -deservedly - and therefore very expensive. (Rare and expensive are sometimes quite unrelated topics when it comes to Canadian paper money.)
I'm sure there are a few - very few - collectors who have a 1931 B of M year set, excluding the $5 variety with the prefix letter S, which is still, as far as I know, unique.
Some of the Bank of Montreal notes are readily available, but there are lots of scarce and rare notes even among the 20th century issues. Common notes would include the $5, $10 and $20 of 1914 and 1923, $5 and $10 of 1931 (the 1931 $10 is common even in AU and UNC, since a couple of bundles have survived), and all of the small size notes of 1935 and 1938. They are seen in abundance at shows and in dealer shops. The 1942 $5 is not as easy to find as was once supposed; neither is it rare. The 1904 $20, all notes of the 1911 and 1912 issues, and the 1931 $20 through $100 are among the scarce notes, and some of these are very rare indeed.
You are right about the word "rare" becoming meaningless. In my opinion, a chartered bank note could reasonably be described as rare if there are not more than 20 surviving examples known. The B of M 1931 $20 fits easily - very easily - into that category.
