More details on our (Toolman's) install.
First of all, our advice from Wrightwood was to "get the biggest unit you could fit in the truck - leave the box behind" so that way you can get an over-size unit and run it on low: quieter, more efficient. Good advice!!!
We were recommended to H&E hardware by Mtn. Hardware and liked their service (we ran smack into one of the managers who took us by the hand, she was excellent). We knew we wanted a side-mount unit and not a roof unit; however, the floor unit (by Champion, a good brand) was too big for the truck (we have a shell). She had mentioned (but not pushed) that they have more expensive units in the storage barn so I said, "ok, how much more and what size are they?"
The unit was a MasterCool II model HC-44 made by AdobeAir, cost was around $350 more than the Champion (including motor); however,
there is a $300 rebate on the MasterCool but not the Champion, so I'm calling it a wash.
I'd advise looking at the SCE rebate page (link below) before you buy - only the most efficient units are on the list. While Champion units are on the list, the ones with the thin pads that you see all over are not efficient enough for the list. Our MasterCool uses a 12" honeycomb-style 'pad' which is more efficient because the whole pad is in front of the blower. There is a cheaper 8" unit (MC44), I don't believe H&E had when we were there. The motor is always sold separately (and more expensive for the MasterCool since the 'pad' is so thick).
Be aware these units are not plug and play, you will need a switch (with pump, low, high settings for rebate), and as Toolman said a duct and register. Oh, yeah and a whopping big hole in the side of the house. Water supply is an easy 1/4" copper tubing that anyone can connect to their outdoor spigot.
After install (and Toolman getting the high setting out of his system), we found that cracking 3 windows about 6" upstairs cooled the house last Sat. from 90deg outside to 71deg inside with the fan on low). Humidity in the house was around 50%.
SCE - Evaporative Cooling
http://www.sce.com/RebatesandSavings/Residential/_Heating+and+Cooling/EvaporativeCooling/