#1 - we did not need converters because our phones and iPads run on either 110 or 220, so that depends upon your personal electronics. Usually it tells you on the plug what it will accept. We did need to use adapters for the plugs.

#2 - conversion rates depend on your bank. We also use Capital One and did not have any conversion fees, so that depends upon your personal credit card. US dollars are widely accepted in Curacao, though change can be given in either US dollars or guilders. It was never a big deal to us though. We actually found having a few guilders was good for thing like parking meters downtown or beach bathrooms.