Clifton - I have the same boat with the same stock trolling motor. Because I wanted to have something for use on my plastic jon boat as well as on the PT175, I have a 55lb thrust Minnkota Vector (12V). I can attest to the fact that 55lbs helps a heck of a lot more than 43lbs! I would echo what others say about getting at least 55 if not 70.
As far as the 24v option, I believe Tee means that the wiring on the PT175 can handle the higher voltage. The issue you may run across as that while you can fit in two size 27 trolling motor batteries and the starter in the back of the boat next to the gas tank, it will be a tight squeeze. You may even need to move or remove the battery trays. Regardless, the three batteries should all fit in there quite snugly. To turn the two trolling motor batteries into a 24v set up, you would hook up the trolling motor wires to the first TM battery, then hook up the second battery by connecting the postive terminal of the second battery to the negative terminal of the first and the negative terminal of the second battery to the positive terminal of the first. Voila! 24 volts.
Good luck!