You'll probably get a better response in the general cleaning section.
Ideally you'll want something that can pump 10+ litres a minute, and a good quality rotary surface cleaner. Also get at least a 15m hose so you don't have to keep moving the washer. Anything with a Honda engine will generally be reliable.
It depends on your budget, available space / weight in van and also how much p/w work you're looking to get. I bought cheap to start with - gradually I'm getting more pressure washing work and I've upgraded as I've gone along, mainly I see p/w work as an additional service for my existing customers.
I recommend getting a turbo nozzle too as they're the mutts
Dave