I have a 16.5 ft starcraft, D star with 12 volt 55lb pd. I figure that's about size of your mirrorcraft. The 55 powered it just fine for as long as wanted to fish for one day. I also have 18.5 alumacraft with 24volt 80lb terrova with ap/spot lock. Moves that much heavier boat around with ease. First, Congrats on getting a new boat, but the alumacraft will weigh more than your mirrorcraft and might require a little more power or 2 batteries for longer time on the water. One more thing to consider, if budget allows, is the higher level trolling motors with spot lock, autopilot, cruise control, are some great features that would help anyone's overall fishing. Think about that before pulling the plug on new tm. I have never heard anyone say,( I wish I had less power.)
Good luck and have fun with new boat.