You can bypass into the supply hose so it creates a circle of water, as long as you don't allow the pump to run for an excessive amount of time it won't be a problem, I adapted my machine to do this 4 yrs a go with no problems.
I would go for the gx390 the extra horsepower lets the engine strain less.
Forget the big FSC they are no quicker you just have to move them slower, and get a turbo lance you will need it for weeds