I've been reading all the replies and everyone forgets some important points.
No matter what you buy, make sure you can eventually get parts or service for the unit as the pump will eventually need rebuilding.
I have had over the years 3 units, one from Northern, from Lowes and my present one. I have a farm and use the machine from washing manure spreaders to doing decks.
My present unit is an Alladain, 4500psi. Its diesel fired, hot water with a 13 horse Honda motor on it.
The average person can get by with a cold water machine but hot water/steam really cuts grease, however, they are pricey. Mine cost almost $5,000.00
If you buy a unit from the local discounter, or Northern or from anybody else for that matter, eventually and most likely when it's out of warranty, the pump will fail and will need rebuilt. I have never seen pump parts, other than complete (pricey) units in a Northern catalog or at Lowes for that matter.
I purchased mine from a local (Detroit area) industrial supplier who also has a service shop. He also stocks chemicals and liquid wax and not the wimpy stuff at Lowes or Northern.
There is nothing more frustrating than going to use a unit and finding out that the pump took a crap and now it's time for a new one and the engine will always outlast the pump unless it's a Tecumseh!
As far as GPM, the GPM is directly controlled by the size of the nozzle orfice. My unit is 4.5 GPM and we run off a hydrant supplied by a well pump. I reduced the GPM flow by going to a smaller orfice at the lance. It's that simple.
