The first question is "How long do you want the motors to run assuming full speed?" Your batteries will need to be ~84 AH per hour you want to run at full throttle. As an example if you have 2 100Ah 12V LifePO4 batteries, you would get ~200/84=2.4 hours. If you build a 12v Battery out of the popular 270AH cells, you would get 270/84=3.2 hours. Obviously, if you are going slower you will have more running time.
The next question is "Build your own" or "Prepackaged" ? Build your own will be cheaper, but you will want to take care to get a good case that is water tight and can take the abuse that it will get in the boat.
If you decide to go prepackaged, SOK might be a good choice for the price, but I do not know how water tight they are.
The best place to put the batteries is be protected area at the bow of the boat (Away from the sun and water), but from a weight point of view, they should probably be at the Stearn. That boat is designed for the big heavy engine in the back so with only the small electric motors it is already going to be bow-heavy.