You’ll likely get different answers from different people.
I started with a Honda GX390 motor mated to a 4 gallon-per-minute General Pump (GP4040). It was a direct-drive unit, meaning there were no drive belts or gearboxes to complicate things. At 4gpm, it had enough flow to operate a small surface cleaner (for concrete pool patios and such). Fine for decks, etc. It wa...