The IIf was originally a lower priced IIIf. It did not thave the slow shutter speed dial on the front. Back in the 1950's Leitz would upgrade a IIf to a IIIf for an extra fee. Thus, there are fewer IIfs arounf than IIIfs. Fewer numbers with Leicas usually means higher collector interest.
The European model numbers were different from the US market model numbers. In the US market, II as in IIc and IIf, indicated a lower priced model to the IIIc and IIIf. The IIf and IIIf were the first post-WWII models. The Russians copied one of the earlier pre-WWII models which was sometimes called a Leica II. In the US market it was known by a letter, either a B, C or D. I can't remember which.
In any event, the IIf is not as common as the IIIf and as long as you don't need shutter speeds slower than 1/25, just as functional.
-Paul