Which minimum isolation level should I use in a transaction which transfers money from an account to another account?

Which isolation level should I use in a transaction which transfers money from an account to another account?

TRANSACTION: 1. BEGIN 2. Check if amount is enough on acc1 3. Subtract the value from acc1 4. Add the value to the acc2 5. COMMIT

Which minimum isolation level is enough here? (If it's possible to do with no transactions just with locks like SELECT FOR UPDATE please mention it, special thanks for that)