Hi Nathan,
The min hp petrol engine you should be looking at to drive a higher flow rated pump would be at least a13 hp engine . Therfore unfortunatly that engine would not be anygood if you just wanted to swap the pump over to say a 15-21 lpm pump. Also might be worth mentioning that decent flat surface cleaners don't really work with lower flow rated pumps either especially on block pavers.
Decent higher rated pumps draw from a buffer tank so yes you will need to use a buffer tank. Even if just using the customers wheelie bin will do if you don't have the room in a van to cart a water butt/wheelie bin etc about. Just make sure you ask the customer first in advance .
Regards to the length of pressure washer hose you use on a job then obvuoisly the more you have to get to the areas you want to pressure wash the less you will have to move the washer and buffer tank about.
Basicaly in a nutshell for cold water pressure washing the higher the flow rate/pressure the washer is set at the less time it will take to clean hard surfaces and inbetween those hard to clean block pavers joints.
Here is a good example vid of how quick a 30 lpm washer can clean block pavers.
https://www.youtube.com/watch?v=sIfOVx6oJJUMessy old job init