I know as well as you do, Andy, that a home pressure washer system is not going to do the job of a professional pressure washer producing lots more pressure and costing a lot more.
but Fred doesnt say he is going into business - it sounded to me, and I may be wrong, that an existing cc customer asked would he do 20 sq yd around his house. A one off? No doubt the customer knew full well Fred was a carpet cleaner and not a pressure washer. 20sq yd is the size of a lounge but in this case spread around the property.
Assuming the condition of this 20 sq yd area around the house was like my block paving then the little home power wash will do a very adequate job (I know that because I just done it) which his customer could be well pleased with.
If its anything more serious then something beefier will be required, so either he goes bigger more powerful, or he turns it down, or sub contracts.
Fred asked for some comments & advice and he has got 3 so far, up to him now what he does with them.