For any of the minor temples, do a search in Panoramio, which gives the location and linked photos, to see if it's a looted jumble of stones or something better. (Many of these are in my spreadsheet, mentioned above.)

Rainy season is fine, but expect transportation challenges, as mentioned above, and bugs.

The Koh Ker complex is composed of 19 sites. They're listed in the spreadsheet along with references to associated Panoramio photos.