It's a bit of a hard question to answer....any pressure washer will remove weeds, although that is not their primary function.
To clean block paving (and remove weeds ) at a reasonable rate so you will make money, I would suggest a minimum of 15 lpm 3000 psi . A turbo nozzle will increase the efficiency of the machine. Weedkilling prior to cleaning would make the job easier. A FSC will reduce the time spent cleaning the surface.