Motor - a 385 or similar run on about half its rated voltage, direct drive.
Prop, 35mm plastic, or use the one from the kit if you figure a good way to mount it. Paint it with Humbrol metallic brass with a smideon of green and white mixed in, let the people ask where you got your brass prop from until the paint starts to wear off.
Shaft about 6-8 inches. That decides where the motor is going when the coupler is taken into account.
A 6volt 4AH SLA will sit forward of the motor, under the forward superstructure/deck.
I did my superstructure in two parts which meant modifying the rigging a bit - a friend doing his has joined the two halves with a strip of ply, probably a much better idea.
Fit a nice high coaming around the hole in the after deck to locate the superstructure, place some styrene sheet as a false deck to give the electronics somewhere to live. The only tricky bit was drilling out the hole for the shaft without damaging the outside of the hull in that area. A very sharp drill, a workmate and lots of patience are required.
I didn't use extra plastic over the hull join, and there has never been a problem with the join. Having said that, with the inevitable knocks over its ten + years of use and (mis)handling it started showing sings of internal water this year. After much fiddling with the prop shaft and rudder mount, it was noted that there seemed to be a few hairline fractures in what should be the bilges in the area of the battery (heavy lump) and the motor mount. These areas were given a coating of Evo Stick PVC Pipe Weld, well rubbed in and subsequently well ventilated. Crossed fingers, this seems to have worked, since the last few calm weather sailings have shown a dry hull inside.
My steering servo was mounted upright on top of the prop shaft under the rearmost part of the aft superstructure - bags of room. You could also make a box for it to lie in under the deck at one side or the other. These things really do not need fixing like model aircraft items, a deep enough box to stop them hopping out unbidden and prevent movement against the forces that they create is enough, unless the intention is to invert it and give it a good shake on a regular basis.
An alternative for batteries is to get 2 of 3 or four cell sticks of D size cells and place them each side of the motor, wired in series. They can be easily positioned for fore and aft trim.
Access is needed through the stern deck to get at the rudder linkage. As the rudder is in line with the prop shaft, and this will need servicing once in a while, you have to be able to remove the rudder to easily withdraw the shaft.
The total weight needs to be about 6.5 pounds, as much as possible as low in the hull as possible, the suggestions above leave leeway for about a pound of trimming ballast, or extra features, like a speaker with a decent magnet for a sound system.
Currently my motor (an ex-ford fiesta window winder) is suspended from a beam fitted at deck level using cord. Its the best aligned and quietest arrangement that I've ever managed in it, and it had its 11th birthday this year.