In temperate climates where freezing is rare, such as here in the SF Bay Area, it would probably be a LOT cheaper just to stick a regular dishwasher in the outdoor kitchen counter - and then replace it if it eventually goes "bad". I would suggest the best options would be the all-stainless models, such as from Bosch. If the cooking are is protected from rain by an awning, then the dishwasher should last for years - or so I would think.
You could buy a lot of $600 dishwashers for $5000...
That is, if you can't stand the thought of shlepping the dirty dishes indoors for a proper dish washing.