Hello Tom
It is advisable to fit strengtheners to the inside of the hull especially at the points of greatest stress, glueing some ribs across the width of the hull will add the necessary stiffness and provide somewhere to fix engine mounts and equipment trays.
I find Araldite rapid to be an excellent waterproof adhesive and it is my preferred glue for building the larger models.
As you will have found the top edge of the hull is very flexible so instead of using 1/4 section timbers to create a connection between the hull and deck I would make a frame from a single piece of 1/4 ply. The ply frame is stiffer than the 1/4 section and will therefore provide a more robust method of resisting the flex in the hull.
The motor sounds ok to me.
I hope this helps
Paul