We just returned from two months in Thailand and took a trip to Siem Reap, Cambodia to see the temples. Well worth the effort, but for seniors in particular be prepared to pace yourself. We had an English language speaker as guide and a driver. We found that early start in the morning with break at lunch followed by rest at hotel until around 3PM worked well for us.
The currency preferred (demanded) for all transactions is the US Dollar. Visa fee for entry at airport is $20 cash, and there is also and exit fee at the airport of $25 cash.