Just a few considerations off the top of my head:

Keep the shaft length to a minimum or your TM head will be sticking out past the end of the deck when in the stowed position. 42" will likely be plenty long enough. I think I had a 36" on mine so I could fully deploy it. Having the TM sticking too far up can be a pain on fig-8 fish as well as providing something else for your fly-line to tangle up on.

Try to angle the TM when mounting it so the head is pulling the center of the boat when deployed, not offset on the side of the deck like many jons I see around here.

If you are worried about theft in your area, you may want to consider purchasing a quick release bracket so you can yank the TM off whenever you want.

Also, make sure to beef up the mounting hardware to the point of overkill. There is nothing more irritating than listening to the TM rattle and clink everytime you engage the motor. They do loosen up over time (via vibrations) if not secured properly.

I would purchase the largest motor available (I think the 55# thrust is still the max for 12v systems). If you have the luxury, go with a 24v system and never worry about windy days. If you are mounting the TM battery in the back of the boat, be sure to use the heaviest gauge wire possible to maximize battery life and performance.