Either the 55/12 or 24/70-80 will work fine. Same battery set up on my Terrova 55 on 17.9 WarEagle. In a real strong wind I have to get it up to 6 to hold. Most of the time when it gets like that it's time to go home. Like someone else said if buying new I would spend the money for variable speed over the 5 speed edge. Much easier to control your speed and batteries will last much longer.
To your other question, yes you can use the same plug, just have to move your jumper on batteries to get 24. Everyone is going to recommend that if your wiring is not already 6 gauge to change it. If it is hard to get to and really difficult to change out your original wiring will work. The place where you would see your biggest drop would be on continues running like pulling cranks. Doubt you would ever see the difference on your boat.