15 litres per minute definitely ok, this will give you more than enough pressure, plus it will work a 18" Rotary Headed Cleaner at full capacity, no problem. If you ever needed more PSI, a turbo nozzle can be connected to your pressure lance.
If it is driveways and patio's you are looking to clean, remember a 300 litre tank will only give you about 20 mins on average cleaning time, so it's important to have option of using customers source of water.