Major credit cards are accepted across Thailand in most establishments, shopping centers, major hotels and large restaurants. Smaller businesses will expect to be paid in cash, as will taxi drivers.
So it is a good idea to bring some Thai Baht along. Foreign credit cards are charged an exchange fee and ATM machines may also have fees. You can change most foreign currencies at money changers in cities around Thailand.
Yes, ATMs are everywhere excepting truly remote areas not easily reached by sealed road.
Most ATMs now accept debit cards as well as credit cards and the CIRRUS and MAESTRO symbols are the most widely accepted. The withdrawal limit ranges from 20,000 to 100,000 baht depending on the bank.
ATMs typically dispense 100, 500, and 1000 baht notes, up to 20 notes per transaction. If you choose to use an ATM instead of currency exchange booth, it is suggested that you withdraw 9,900฿ instead of 10,000฿ (for example) as this forces the ATM to dispense several smaller denominated notes. Be aware that most ATM's now charge 150 baht for all withdrawals using foreign cards.
Visitors entering the Kingdom on a tourist visa are entitled to a 7% VAT refund on goods purchased at registered retail outlets and Thailand duty free stores. The VAT refund may be claimed on a minimum total of purchases worth 5,000 baht from receipts totaling no less than 2,000 baht/receipt/day.
After purchasing goods at a store, visitors must fill out VAT refund paperwork at the store, providing passport and travel information at that time.
At the airport, paperwork must be presented to customs officers before passing through immigration at the VAT refund office; at Suvarnabhumi the VAT refund office is near the entrance to Domestic Departures.
It depends a lot on what type of traveler you are and how much travelling you plan to do.
If you're eating on the street, three meals a day, then 200-300B per day is adequate. If you're eating three staple meals in a backpacker style restaurant, you'll be spending more like 250-400B a day. If you're eating dinner at proper restaurants, then you could easily be spending 300B or more a meal.