Hi Bill
I'm using the ones that Slipway supply as a motor fittings kit. These are Torpedo 500 motors and are rated at 4.5---15volts so will handle 6 or 12 volt comfortably. They are geared down at 2.5 to 1.0. according to the label. I'm using them at 6v as I already have a SLA and various other 6 volt units in my spares box which I'll use up on this model. I am not using it for towing so I'll try and make it look pretty instead.
The power is not important to me for this model and the motors have offset shafts near to their bases. The prop shaft is fairly tight to the bottom of the hull so the motors and shafts are a well matched combination. I put the motors on to thin ply bases and the couplings were smack in line.Phew, that's lucky because it doesn't very often work out that way. Anyway, I'm saving my cache of Decaperms for a rainy day........................
I'm also going to try the supplied props before I buy Prop-shop, or similar, to see what happens.
Hope this helps a little.
Best of luck.....................