Interesting.....
My 12yo, wanted a model boat.
I got a crappy but complete Aerokits 'Sea Scout' of fleabay , £17 delivered and took it from there. It came with motor, prop and rudder.
I've stripped the paint, re-sprayed using paint from the Carboot at a £1 a can, the radio is old stuff lying un-used, ESC is out of a Model car at the boot as it the battery and charger. I've had to get a new coupling and do a bit of 'modelling' in the internal bit - total cost at the moment under £40
She's watched every step of the re-build, and thoroughly enjoyed it
Best of luck with your challenge
Cheers
Mark