As for the hp of the motor to use, I have a 6hp Nissan 4 stroke on my 22ft Trophy that does exactly what I need it to do and more. I can easily run at trolling speeds into the wind. I did put the sailboat prop on it to get additional thrust. It burns hardly any gas. It has saved me alot of money as opposed to running the big motor with bags to troll.

Mounting should probably be done on a good Trolling motor mount to the transom and reinforced with marine plywood, or like I do, a thick piece of plastic cutting board on both sides. It will never rot. You can get one from GFS, they are about 1/2 inch thick.

***Be sure to measure correctly when mounting your bracket***

I would also say that a long shaft (20") is likely your best bet. The 15 inch short shaft may have a hard time staying in the water with waves. Mounting it too low to make up for the difference risks submerging it in the waves.

Also be sure to size the mount to fit the motor you choose. I went with one that could hold up to a 15 horse 4 stroke.
You can also hook the 4 stroke straight to the main tank and run raw gas.

The next step up is an 8hp... The cost jumps alot from a 1 cylinder to a two cylinder engine as does the motor weight.