Both US dollars and local currency Riel work well here. You can withdraw from ATM with your bank card, it offers both Dollars and Riel on ATM. There are some (not really many like HK) exchange shops in the city and airport as well. So generally currency is not a problem at all here.
Just remind that if you hold Chinese passport then you must wait one month and half for visa after you deliver documents to Thai Embassy in Cambodia. Not sure if there's something different with Thai Embassy in Sri Lanka and Philippines.
It should be that every Thai Embassy in every different place(country) has different policy to apply Thai Visa, I think now everyone no matter where you are from but if you apply Thai visa in Thailand Embassy in Cambodia, then you need proof of 1000 US dollars. I guess there's no need economic proof if apply in some other countries like Japan, UK, Germany...