Summary of the items I learned here and other sites when I was shopping for one 18 months or so ago:
Get one with a name brand engine that you can get parts for locally - pick your favorite flavor.
Triplex pump is preferred to axial pump for durability and longevity. Cat is supposed to be really good stuff if you can find one in your price range.
Pressure is what breaks bond between dirt and object being cleaned while volume determines how quickly the work will be done. You want a decent balance between the two. Some sites use a term of cleaning units (psi x gpm = cu) to show total cleaning ability relative to one another.
Soap tanks just give you one more thing to clean when your done working / cleaning so soap siphon is preferred by many. After using mine, I would agree completely.
Longer hoses are more convenient than moving the unit all the time with a short hose.
Storing them in a heated garage or basement is ideal but treating with pumpsaver / antifreeze is a must if you can't do that (unless you really want to replace the pump).
With all of that in mind, I ended up with a
Simpson ALH3225 from Lowes for $499+tax. Kohler engine is supported locally by a couple of equipment dealers and has run great since buying it in August 2014. 3200 psi and 2.5 gpm is plenty for my uses around the house / yard and cleaning undersides of cars at the end of winter. I added a 25' extension hose to the stock one for up to 60' of total reach. Also added quick connect fittings for the water inlet, pressure outlet, hose ends, and spray wand - I'm all for saving time whenever I can. At some point, I would like to add a surface cleaner for use in the garage once or twice a year without getting over spray everywhere.
Two negatives of the unit I've found so far is that the handle is pretty short even for me at 5'-10" and I'd like to be able to mount a gallon of cleaner on it so I don't have to move multiple things every time I move the machine (think about cleaning a fence for example).
