I use a Karcher K5 from a couple of years back, before they started putting electronic controls on the lance handle. The best feature is the relatively quiet motor and pump, and the long highly-flexible hose on a reel. I also use a tiny little Karcher K2 as a mobile unit - same fittings - which works about half as well and the hose is annoyingly short and springy. The K5 is worth the money!
Because I need the unit to run for a minute, stop for ten minutes, start for a minute, stop again etc. I can’t imagine having to use a petrol-powered unit; they’re for cleaning houses and fences, I reckon.
I wash cars in a slightly unconventional way to avoid using any buckets, which cause me grief. Two-bucket wash never worked for me as I’d always dip into the wrong bucket, plus, buckets get kicked over or left out to deteriorate/blow away etc. and it takes time to fill them, and, many solutions seem to lose their suds so you can’t see where you’re washing.
I spray the car with foam - a little goes a long way with a good-quality bottle sprayer (I use ‘PA Italy’ sprayer and ValetPro foam).
Then rinse the car with high-pressure lance, to remove most of the dirt. This is the non-contact part of the wash. In really dirty situations, a citrus foam and rotating nozzle works best. However, there is always a thin layer of dirt left behind, even with special ceramic coatings etc. that are supposed to make cleaning easier!
So then I attach a second bottle to the sprayer which is filled with the ‘coating’ wash solution (Fireball Hydrophobic Foam, or Gyeon Bathe+, or similar).
I spray coating foam on one side of the car and detach the sprayer from the handle.
I then use a mitt to clean the car, using the ‘coating’ foam applied, rinsing the mitt using the spray from the handle without lance attached. No areas are missed because I can see where the foam hasn’t been wiped away. A benefit of the K5 is that it runs in a slow, gentle spray when no lance is fitted. I like a bright-coloured mitt because I can see when it’s clean. I use an older mitt for the wheels. No buckets...
I attach the high-pressure nozzle and rinse the car, then repeat for the other side. In cooler weather the whole car can be rinsed in one hit.
The high-pressure spray disperses the coating solution effectively and a lovely hydrophobic result is achieved on windows and trim in particular, making drying with a towel easy. I use a Ryobi 18v cordless leaf blower in conjunction with the towel, blowing water out from mirrors, wheels, lights, base of the windscreen, etc. - the towel is still needed as otherwise I’d be chasing drops all across panels.
The Karcher fittings are a bayonet so it’s quick to change between spray bottle and pressure nozzle (and, to use the handle by itself).
Detailing is my trade at the moment, so I wash about 3-5 cars a week. I also use a ‘washworld’ facility in town for this, but I take my own mitts and use a similar process, just with their foam sprayer and high-pressure rinse (and never their ‘broom’). It seems to go a lot faster when I’m having to feed a machine with coins
-Alex