I agree to go with a 14-71 as opposed to the 12-71, for your size motor. The 14 is 1" longer.
You do not need the expense of a high helix for what you are doing, but if you can get a deal on one.....
A regular ol, 14 is fine, and there is not a whole lot of difference from bds, littlefield, etc.
For a used one, you need to make sure everything is ok inside, no damage, none of the strips coming apart or that anything has gone through it. You would be suprised at some of the junk I have seen , where guys try to tell you it is ok...and on the internet it is more difficult. Try to locate one within reasonable driving distance. Buying a bad blower cheap will cost you more to repair it that it is worth.