I guide and fish out of a 18' lund. I have a 90 4 stroke merc and wish I would have gone with the 115. I feel you should always max every thing your boat can handle. I'm on my third trolling motor. I recently put a 101 terova on my boat and couldn't be happier. If you max every thing out you will never be left wishing you had more power when you need it.

As far as aluminum. I regret not going with fiberglass. Don't get me wrong I love my boat but afer running my buddies ranger for a week I realized how much easier boat control is with a glass boat. My next boat will be glass.

Hope that helped a little. Good luck.