You have the right troll motor picked out. As far as usage time it will depend on the speed at which you use the troll motor. At high speed, you may only get an hour per charge, half speed will get you about two hours, low speed maybe up to 3 hours. Normally you will get tired of fishing before the troll motor goes dead! If you need to run a little ways to get to your fishing spot you are better of inveting in a small outboard.
As far as the battery, go with a good name brand, that will get you longer and better performance. If money is an issue you can go to wally world and get a cheaper battery but it will not last as long. They do have different size troll motor batteries, the bigger the longer you will have power. But be carefull bigger is not always better. If you don't need that much you will not draw the battery down enough to properly recharge it and the bigger the heavier!
Finally, the charger, I use to have expensive charges with all kind of options on it, not any more. I use a very small compact charger that I got at harbour frieght for 20 bucks and it works great! We also use these at the dealership.

I hope this helps,

Rich