Its best if you carry THB cash and your debit/credit card as people prefer payment in cash in Thailand. Also, you'll be charged with the same conversion amount (THB 200) if you take out money from an ATM through your Forex or Debit/Credit card.
For visa on arrival, its required that you carry THB 10,000 but they usually do not check it. (Just the passport, hotel and return flight documents).
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