I bought an AA one off the bay for around £50 with the patio cleaner and dirt blaster type lance, doesn't do a bad job. It probably won't last as long as a more expensive water cooled (on some models) Karcher but it was cheaper to buy a cheap one than to hire one as it has already paid for itself. If the AA one hadn't been around I would have gone for a Karcher, probably a K2 for value for money and probably from Ideal World as they have some good package deals at times. I used to sell Karcher many years ago when I worked for Halfords so I know you can get some good deals at times as the accessories can cost quite a bit to buy in a retail outlet.
For cars a K2 will do just fine but try find one that comes with the wash brush as the detergent once applied to the car works best if agitated with the brush before jetting it all off.