OK - first off for general car cleaning and the like the K3 series is as far as I would personally go. The K4, K5 and upwards will do the job marginally quicker, but Hey - we enjoy cleaning the BMW, right?
Here's the cautionary bit. If you buy a machine with a dirt blaster - that's the lance that looks like it has a walnut whip on the end LEAVE IT IN THE SHED. You do not and will never need this accessory for the car. A high pressure lance is all you need - that and a brush of course.
I would never get closer than 18 inches (450mm) to my car with a HP lance - give special caution to parking sensors and also to tyre valves, these are the most sensitive areas on your car. Also be careful cleaning under wheel arches - don't be tempted to poke the lance under the arch and wave it about. The paint inside wheel arches is NOT the same as exterior / bodywork paint.
I hope this answers your questions so far Guys.