I wouldn't expect long term reliability from a $400 petrol pressure cleaner ... Obviously depending on level of use.
I've got a mid range electric Karcher and it's the one electric tool I have that I'd be tempted to swap for a petrol variant. In Oz I'd be looking to spend $1,200~$1,500 for something decent, so I'd be expecting a $750+ price point for a reasonable high pressure petrol unit in the U.S.
Also think about accessories ... this is where you get stung!! Buying a complete kit that suits you, with long enough hoses, etc is a smart move. Most domestic units won't even get you round the average car without moving the unit because the included hoses are too short.
Last little point ... blasting timber with high pressure can really cause damage to the timber. Torture a scrap for a while if you have any doubts [wink]