Where did the 500’ sand bar thing come from? If you want a 500’ utility airplane with respectable cruise maybe look to a Husky or a 180. Good STOL performance, constant speed prop, fast. And if you want to take some stuff? Advantage 180. The slats-split flaps Cub will shine when you want to operate on a 100’ sand bar.
Most guys with a cub can land in 500' without any issues... if not, well, you need practice.
500' in a husky takes a guy that that can hold + or - 5 on final, no big deal.
500' in a 180 you better have your $hit together, know the plane inside and out, and be on top of your game that day.
I have done 500' with all three, and I can tell you things had to be perfect with the 180, (I flew that plane over 1000 hours and knew it well).
Every modification, option, engine and item you put in or on your plane has a benefit, and a penalty. Weight, cost, complexity, usability... something changes. Before I started into a $100,000+ project that would take two years of my life or more, I would be seeking out guys like Stewart and Bill to go flying with them and see what/how they created their dream machines and learn what I wanted or did not want.
Talk and posting over the web is cheap, but you should fly some of the modifications before you commit; 90% of your flying will be doing what? That is what to build for.
My opinion and experience, worth what you paid for it!!!