Important things to consider
From our point of view, we want a pressure washer that has good PSI (2000 or thereabouts), good water flow (from 7-9L/min), and a long hose (10m if possible). A braided hose is also a big plus, and a hose reel to wind the hose back up on is extremely helpful. Keep in mind that the hose can be TOO long. A 15m hose can become tangled up on itself, so we find about a 10m long hose is perfect. You want to set the machine up at the front of the car, and start snow foaming from the rear of the car.
In terms of the actual machine, the sweet spot for creating mega thick snow foam with the Snow Blow Cannon, is around the 1800-2300psi range. You may also want to consider things like what extra attachments or features the unit comes with; like an attachment for cleaning the driveway, getting under wheel arches etc.
The weight of the machine is also one to consider. Think about how far and how much you'll be moving the pressure washer around - whether you need to lift it over anything, or up stairs. A 20kg pressure washer can become very cumbersome and heavy to cart around.
Electric vs petrol is another point of contention. We prefer the electric machines as the petrol machines are often extremely loud and can require more maintenance than the electric models, not to mention you will have to put petrol in them to use them. The petrol ones however can be more powerful, so be sure to take that into consideration when picking a machine.