I have used a karcher before on a small, high balcony clean that needed an electric PW in cherry picker would have been to costly.
It did the job on slabs, but they were relatively clean really.
For block paved drives, getting weeds out etc. it is the wrong tool for the job. As Kevin said, its not a professional job. The quality would not be good enough.
Why not sub the work to a local company and take a cut for doing nothing?