Rainy season in Cambodia is from May to October and it is best to avoid this time for a visit to this location because getting around with flooded roads can be a huge challenge here. November and January are the most ideal vacation times here as the weather is quite cool and pleasant.
You can get your visa when you land in the Kingdom either at Siem Reap or Phnom Penh. However, for entry from certain points, visitors need to have a visa ready. Online visa processing is possible and you can also get further information about these requirements from the Royal Embassy of the Kingdom of Cambodia located in their country.
The ubiquitous use of the U.S. dollar here is a great convenience although the Reil is the official Cambodian currency. Keep in mind though that Riel is not convertible so if you do opt for currency exchange into this currency do so with care. Keep well stocked with cash and travelers checks because credits are not accepted everywhere.
World class medical facilities are not easy to find in Cambodia and even the number of pharmacies is rather limited. Getting immunizations for malaria, thyroid, tetanus, cholera, TB, hepatitis A/B before you leave is a great idea. Carry your own medicines for relatively minor problems like diarrhea, cold and fevers and you should be able to manage just fine.