If using the kicker on a dinghy there are many good reasons to stick with a 15" shaft. If only for a kicker--then a 20" shaft. The 15" shaft does not get the kicker below the hull--it will work to push the boat forward, but does not work well in reverse, because the prop wash pushes against the hull. You want to keep the power head out of the water/waves as much as possible, and the 20" shaft helps with that in heavier seas.
The Tohastu is an excellent choice. My Merc- (actually a Tohastu) has only the small tank--it will run a long time at medium throttle.) Having an alternate fuel tank will make it much easier than pouring fuel into the integral tank! (as the 4 HP on up can have).
As for steering I use either one of the clamp on articuilated tiller extenders, or just slip a piece of PvC pipe (which you can modify to get around any obstructions easily) over the tiller. If you want one 4 feet long--no problem!
The battery charging might be handy, but it is only 5 amps!-- I prefer a Honda 1000, with a good charger (like the West Marine portable 30 amp, which has multiple settings, and compensations, plus a trickle charge and engine start boost function. However, this combo is going to cost around $1000 vs the $400 for the add on alternator..But it is far more versitile, including running a 5000 BTU air conditioner. (so much for minimalist)