Visitors from an ASEAN member country do not need a visa to enter Cambodia, as long as they have a valid passport. If you need to obtain a visa, visas for Cambodia are available at airports and land borders. In order to obtain one, make sure you can provide a passport valid for at least six months and a passport-sized self photo for your visa.
Citizens from the following countries will need to make advance arrangements for their visa: Afghanistan, Algeria, Bangladesh, Iran, Iraq, Nigeria, Pakistan, Saudi Arabia, Sri Lanka and Sudan.

Be aware that overcharging can be an issue at land borders with Thailand and Laos. To avoid that, you may want to arrange a visa in advance.
Online e-visas are only accepted at Phnom Penh and Siem Reap airports, and at the land borders of Bavet/Moc Bai (Vietnam), Cham Yeam/Hat Lek (Thailand) and Poipet/Aranya Prahet (Thailand).

Dry season runs from November to April, and temperatures are at their most comfortable from December to February (which is around 25°C), with the least rainfall in December and January.

Yes. Now Cambodian government allows tourists to ride rental motos. Keep in mind, though, that you would need a Cambodian driver's license, not an international driver's license. The local license can only be obtained in Phnom Penh.

You may get better exchange rates at local markets. Try and spot shops that show foreign money in the front. Exchanging small amounts at a time is ideal since you can use US dollars at many places. Keep in mind that if you pay with US dollars you might also receive Riel in return, so try to not use bigger banknotes.

It is not necessary to give a tip in Cambodia. However, a small tip at your discretion when you receive good services can go a long way for workers.