You lose horsepower as you go UP due to the density of air being less. The formula is 3% loss for every 1000 ft. above sea level. With the difference getting pretty noticeable above 3000 ft. So, if performance is your main concern, the 20 is the obvious "best" choice. The three outboards you listed are close enough in weight to make that so.
But the price point of the 20 is quite far, in dollars, from the 9hp. If money is anything of an issue, then there's that.
As far as changing the prop, you would need to factor the type of load you'll be moving more than anything else and it's likely the stock Honda prop on any of the three outboards will be adequate for 90% of most tasks asked of it.
Where you will probably need to make a change for altitude will be with the carburetor. Since the air at 7000 ft. is so much less dense than at sea level, you may want to look at having a smaller orificed main jet to keep from running rich all the time. Honda sells different sized jets for these carbs for that very reason.
However, should you bring the boat down the hill to run at sea level, you would definitely want to install the original jet so as not to have the engine running too lean. Lean burn can damage an engine, especially on a hot day.
My approach would be to run it stock at first and see how it goes. If you don't have a big problem with black exhaust or black sooted and fouled plugs then I wouldn't change a thing.
If you do find that she needs to be leaned out a bit but you still want to take her down to fish the bay seaside, I would buy a spare carb and have the proper jet installed. Then a quick swap. and off you go.
Good luck