One to 500 is the minimum we recommend rather than a set guideline for perfect power. In fact, the only negative user report was from Scott(Audeojude) who was using the 6 on a load that exceeded 3000 lbs...Hence his issue with the engine reaching it's rated hp. When he purchased that 6(from another dealer) it came with a higher pitched propeller. The "pig" propeller was not available for sale in the U.S.(except through us) so he ended up wasting a lot of effort trying to fix an engine that wasn't broken. At last report, he was satisfied, though not thrilled with the engine.
Personally I like the 6, if it is utilized for what it is intended. Boats up to 25' and under 3000 lbs. I like it even better when that load is under 2500 lbs as the closer you get to 3000 the more likely you are to be in a situation that a single cylinder 6 can't handle. People also tend to forget about the windage that a hull can have, which can also work against a small engine. So, I would tell you that you really should go with a two cylinder, 6-9.9, in any brand, as a first choice. A 6 Sail Pro will work for you, but there will be times where you will wish you had more.
The Johnson would be a bad choice due to it being a short shaft and 15 years old. In it's day it was a great engine, but anything over ten years old does not meet the requirement of dependable power. That crosses every brand and model line. Lot's of individual exceptions to that...But, that is the reason they are exceptions
Current engines I would look at would be the Yamaha 6/8 and the Tohatsu/Nissan/Mercury 8/9.8/9.9. I prefer the Tohatsu, but I am self serving