One option you didn't mention (probably bc it just came out) is jackson's newest, 'The Big Rig'. From what I can gather it is very similar to the Big Tuna, but slightly scaled down for one person and with upgrades
http://www.youtube.com/watch?v=937Nyb886hw
I would not get a tandem. All that means is you are going to have to peddle or paddle something built for 2 ppl, solo 95% of the time, although you did say you would also run a motor. Supposedly the Big Tuna still does well solo but I wouldn't.
Not sure if you saw either the PA or Big Tuna in person yet, I have. But they are much closer to boats then they are kayaks (not a bad thing, just they are large). Just mentioning bc you said your shoulders aren't the best loading/unloading.