Excellent build to start a blog with here
in regards to running hardware and electrical...
models 31" long so to be honest won't be that heavy!
there are two ways to go...
i'll start brushed motors first as that seams to be your plan.
boat can probably take easily a plain 40 or x40 prop ( x40 will make boat go faster and the incress of amp draw)
Suggest plain 40 prop, on a standard 540 motor with an mtronics 15 amp ESC plug and play, no messing around with setting up start and stop postions!. ( forward and reverse with built in BEC) as power with either a 12v slimline Lead acid battery or a 7.2 or 8.4v rc car pack. ( recommend 8.4v pack just for the extra RPM)
That will get the boat moving at a reasonable over scale speed for about 20-30mins on the rc car pack or if using a heavy Lead acid, upwards of and hour +.
you can change the motor to a 600 size with the same size ESC and battery option. ( if you do suggest swapping to the X40 prop) but battery time will be reduced. ( suggest graphner speed 600 8.4v, there is a 7.2 but may as well go 8.4v even if you run a 7.2v pack)
That setup will cost you around £30 with prop (excluding battery pack) £10 for the motor £20ish... for the esc ( varys where you buy it)
If you want to go like a bat outta hell and 31" boat with either setup will be fast regardless of weight ( as long as its not over 15kg!)
then you can go brush less.
I'm currently building a boat of similar size ( its a cruiser too ) which runs a 1500KV brushless outrunner ( made by overlander, only true british made motor that i highly recommend even tho a bit pricey), and a 30 amp watercooled ESC on a x40 prop, and running on a 3s Lipo. (11.1V) Warning
here tho many brushless ESCS don't do forwards and backwards easily even with program cards ( no matter even if the say they do!), so take this into account that your model may only ever go forwards with this setup!
suggest reading the below linked build as it contains videos with proof of the running ability's) https://www.thercmodelboatforum.com/t1044-aero-naut-jenny
that setup will get that boat flying!!!
cost for that setup will be around £25 for the motor and £30 for the ESC, you will
need to add water cooling to the boat to ensure the esc dose not over heat!
The lipo price varys, (check out hobby king for their selections) you want a lipo that is 3S with a discharge rate of at least 30C, size of battery around 3000-4000mahp but you can get them starting at £15 up to £50! (excluding a charger)get back to us on the prop size you will be using regardless of my recommendation ( as a larger or smaller prop may be required depending on your build and i'll suggest something from there, but i expect a 40mm prop will be the best way to go giving you options!)
Choice is yours on what you want to do, scale speed i do suggest brushed motors ( first set of suggestions) as that will give you decent get out of trouble speed and you can throttle back for more cruising/scale appearance.
Talking on appearance you can add a water cooling coil to 540's and 600 sized brushed motors if you want the water cooled effect on the boat its not only more realistic but also practical use as brushed motors do tend to get hot after longer runs at lower speeds.
Brush less motors tend to like going fast rather then slow so do take that into account before deciding.