The problem with Hotsy, and Karcher is that everything is proprietary. That means that you have to go to the dealer to get the parts. There are a lot of brands out there that use non-proprietary parts. If you are going to buy a large pressure washer, I would recommend one of those.
As for washing cars with a pressure washer, as with anything it depends on the nut holding the equipement. If you get the larger pressure washer, you can use a larger tip to reduce the pressure. You do not want to adjust the unloader!!! That is like choking off the carburetor to make an engine run slower. It will work, but it is really hard on the equipment. The advantage to getting the larger pressure washer is that you can also do other types of work with it, and quite often it is much more profitable than detailing.
Scooter