theres no way you need to spend 500-700 to get a decent jetwash! 
I'd be scared of the paint getting stripped with something more powerful than a typical £100-£200 karcher Pressure Washer 
Once you try a proper one there's no going back haha.
I regularly use a coin operated industrial series Kärcher at my local DIY garage for pre-washing and the difference to the basic 200£ Kärchers is just out of this world. I did manage to wash the balancing weights of my wheels once when I was too enthusiastic with the washer.
It's not the pressure but the water flow (combined with pressure => power) that sets the washers apart. The pressure is adjustable on the better machines and you're usually fine at around or less than 100 bar. If you set the pressure up to eleven on the more powerful machines and wash up close you can of course do damage...
