I have recently bought a refurbished Karcher 550M from these guys:-
http://www.lloyd-jones.com/store/cleaning/cwpwd.html
It looks to be quite a good model for the money with a proper steel pump that should last a while. Pressure and flow-rate are good, more than enough power for car washing. It also has a built in soap container so you don't have to mess around with soap attachments.
M.