Sorry I can't help you with the length. But I know that once you have a TM, you will not have a fishing boat without one. It will be a great addition for fishing the Kennebec, especially if you fish the deeper spots. I've had mine for 3 years and I haven't touched my anchor in that time.
I'm guessing you'll need at least an 87" shaft and a 36V. I've got an Ulterra but a lot of guys are happy with the Terrova as well. I can't speak for the other brands but everyone has an opinion on brand.
Do you trailer or slip your boat?