G'day gman,
I'm a bit of a fan of these brushed motors, and have been using them since 1982. I did a bit of an input under the Albert blog on this page. Albert uses a smaller motor and direct drive, but performs very nicely and almost silent running after initially trying a very noisy and troublesome brushless setup! Used the same arrangement in my little 1/16 scale Dredge Tender that I built while waiting for Albert to turn up.
My first tug, the Sydney based "Iron Cove", built in 1982, is still running the original motor and drive with a 6 inch x 4 blade prop. She's 58 1/2" long x 16 inch beam and carries a lot of lead ballast.
It uses a 12v fan motor out of a Ford ( Falcon ), with a 1/4" round section urethane drive belt ( Redthane belting ) on home turned pulleys at about 3 - 1 reduction.
Runs all day + on a car battery. The Esc is a unit that used to be made by a member of my club back then, but now I use the cheap car type ones readily available on the internet for very few dollars.
Don't have any images of this build, as it was a long time ago, but if you want an image or two of my drive system, I can do that in the next day or two.
I'm guessing you are building a shallow draught pusher with not much hull depth, so depending on how much room you have, you could maybe use direct drive with smaller diam props. The old rule of thumb use to be, that you used a motor of a similar diameter to the prop.
There are plenty of people on here that can probably give you far more technical detail than I can, but what I do works for me, and has done for a lot of years. I like things simple!
Jarvo beat me to posting my reply, but what he says is pretty right, and my Iron Cove also pulls a dinghy with no problem.
You could get away with only using one rudder servo if you use a Hi torgue metal geared unit.
Regards
Ian.