I recently purchased a Minn Kota Traxxas. It has a 42" shaft, 55 lbs. thrust, and is a variable speed model. I didn't have my bow mount ready last time out so I used it on the transom. It is harder for a TM to "push" than it is for it to "pull". Unfortunately, I didn't have my hand-held GPS with me to know the speed, but it was doing everything I would want in a TM. I had the motor on my 1754 Tracker Grizzly with a light load. The boat itself is just over 500 lbs..
The variable speed should give you longer run time because you do not always need all the power/thrust/speed that the typical 5-speed offers. I have read that the "Digital Maximizer" does help, but I don't know if it keeps you on the water 5x longer. But hey, if they say it in a catalog, it's got to be true... right? It's impossible to give an honest accurate answer without more details on your boat hull shape, width, length and such. What I can say is that if you plan on trolling for any length of time (even in calm weather), your battery may not be up for the task. If you can use your gas motor, you may be ahead of the game. With larger motors you have the option of using a trolling plate to slow you down.
I've seen charts somewhere before that recommends certain minimum thrust lbs. for certain size boats. Maybe someone will chime in and post a link to the site. The "folks-n-the-know" recommended 50 lbs. of thrust for my size boat I think... but a lot of it has to do with how you plan to use it and where. There is a big difference between scooting around in a protected cove verses being out in the middle of a big lake fighting waves and headwind. Just things to consider.
