The key to pressure washer performance is a combination of pressure (bars) and flow rate (litres per hour). Imagine the fireman's hose - literally thousands and thousands of litres of water per hour but at 3 or 4 bar. You need good pressure - 120 - 140 bar and a decent flow rate - 350 - 500 litres. For the old school a bar is equivalent to 14.7 psi. Note that there is a difference to maximum pressure and rated pressure. Make sure when comparing fruit it is apples with apples.
Karcher pressure washers are sold in series - much the same as BMWs. They start at 2 series, denoted by K2*** (stars being model numbers, ie K2300, K2385, K2400, etc) through to 7 series, denoted by K7***. There is an 8 series but this is a hot water machine and not widely available in the UK.
The K2*** machines are opening price point and cover light duty work. For the serious car owner, with drive way, garden, patio etc, the K3 or K4 should be ample. There are a number of K3*** models out there, most are actually made in Italy rather than Germany - Karcher own a couple of factories there. There is also one K3* sold through Screwfix - made in China.
Look out for the "X design" - the latest range available for K3*** through to K5***, i.e. K3500X. These have water cooled motors are are therefore quieter to use.
B&Q have the only K6*** on the market, the K6.160, and these are on sell through right now. You can get the only K7*** from Makro if you have a trade card - K7280. This has a brass cylinder head so should last for ever.
Also conside the length of high pressure hose that comes with them. 4 metres for the K2 makes them cheap but means having to continally move the machine as you wash the car. At the top end you are looking at 9 - 12 metres!!!
The higher you go in the range the more cleaning applications are open to you. Look out for detergent handling capabilty, starts with a little tube that "sucks" detergent through, mixes with water and is applied at low pressure, either through the lance or brush. Higher specced machines will have on board detergent tanks - maybe even twin tanks.
Any questions, hehehe