For doing driveways/patios you will want 3000psi and 15lpm, I would always buy honda engine if you're using it professionally. The one omnipole sell is the right spec for domestic driveways, have a look at dual pumps; jetwash direct; malcleanse have a very informative site. The above is over spec'd for guttering so it depends on what your going to use it for most, any cheapish pressure washer would probably do the guttering, the smaller ones can run off an outside tap where the larger versions will need a feeder tank to suck from.
Neil