I you have not done so already, you might want to look at this recent thread, even if the OP's question wasn't the same as yours, some answers may prove informative to you too.
http://www.dr1.com/forums/living/103537-c-whole-house-options.html
The "good" or "reputable" band in split and thru the window units is Mitsubishi... and
maybe some lines of LG too. Mitsubishi is expensive though.
The salty air, humidity and poor electrical service may however strain such a good quality machine too. The question then is... is it, in this country really worth "investing" in expensive quality if the environment threatens to get the best of it anyway? I can't answer that question for you.
Most dealers who sell reputable brands will be as reputable as they may come around here.
On the other hand... for no-name brand A/C's I don't dislike PriceSmart. Sometimes they DO have very competitive offers and the have somewhat of a customer's service. They also have inexpensive thru the window units.
Be sure to "secure" your units! Outside (compressor) units tend to become attractive targets for "opportunists", so do thru the window units, with the added risk of offering entry to your home. You need to have a steel bar protection custom made in some situations.
... J-D.