the biggest difference from your home depot/sears garbage and commercial washers is the flow rate. most of the cheapies are running about 1.5gpm. the commercial landa units we rented at the yard i used to work at ran 4gpm.
our equipment washing rig was a 4gpm 1500psi electric washer. we rented 2000, 2500, and 4000PSI washers that were gas powered. the 2000 had a 7hp honda, 2500 had an 11hp honda, and the 4000 had a 16hp briggs V-twin. with a standard 15* tip the briggs will easily take paint off a car.
however the performance comes at a price, if you're buying new expect to pay around $1/PSI. used you can pick up a 2500 for around $700.
also, stay away from direct drive units, beltdrive is the way to go. the direct drive ones put more load on the motor and usually don't last as long.