2.5 GPS can be on the fast side depending on the baits you're fishing, especially if your going with the current and waves.
I also wouldn't be out there without the second motor, the CG will not come out and retrieve you unless you're in "imminent danger", otherwise they will tell you to phone a friend or request assistance from another boater.
Staying in the 24 volt (without adding a battery) you'll only be going up to 80# thrust. You will not troll very long on a 24 volt motor at Lake Michigan speeds. Spot Lock and Autopilot are great features when you do upgrade though.