Seriously though folks😁
there are dozens (hundreds?) of variations on the 540/545/550 theme 🤔
I have a pair of original standard ancient Mabuchi 540s in my 1.35m destroyer and she goes like stink on 6 to 12V. Makes a lot of RTR plastic "speed boats" look downright daft😊
Once upon a time the 545 was simply a 5 pole version of the cheap n cheerful 540, which gave smoother starting and better low speed control and torque.
Nowadays there seem to be endless variations of both 540 and 545☹️
Very generally speaking the 540s seem to be higher revving and the 545s seem to have more torque. So look at the specs / price before you buy!
More important is the size, displacement (hull type) and probable weight of the boat you want to shove around.
For a tramp steamer obviously a displacement hull designed to carry weight so I would go for a low revving high torque version.
Re shaft and prop; again hull type, size and weight of the boat are important factors.
More so than the length of the shaft.
In my 52cm fish cutter I am using a 3mm shaft
and a 30mm prop.
Much bigger / heavier than that (or higher revving motor) I would use a 4mm shaft
I have some 1.5m warships where the load is shared by 3 or 4 shafts so they also have 3mm shafts
and 30 or 35mm props. Shafts are also about 10 to 12" (25 to 30cm)
What size prop is stated on your drawing?
The larger your prop and the more power you are putting into it the more robust (thicker) the shaft will need to be.
My 24" Sea Scout, with a powerful brushless motor and about 260mm shaft has a 4mm diameter shaft and 35mm prop.
PS Another example folks of where not all the necessary info is given to enable a definitive answer!