I made the trip in a 19' Center Console with a 150HP outboard. The boat has a 55 gallon fuel tank and I took an extra 10 gallons with me "just in case". I was with a couple of other boats, a 22'er and (2) 24'ers. Got there in just over 2 1/2 hours and could have been quicker if the fish hadn't started biting! I still had just over a half tank of fuel left when we got to port. You will need a yellow quarantine flag to fly until you are cleared by the port authority, you must show registration and insurance.
We prepped the boats the night before, and headed out of Miami at the crack of dawn; we were cleared by customs and sippin' brews before lunchtime. The water is soooo clear, and the fishing is amazing. The locals are very relaxed; beware you can easily get caught up in "island time", basically why do today what you can put off until tomorrow.
All of our boats were in top-notch running condition, we all had VHF and GPS, and we checked and rechecked the weather as well as the buoys, which you can do by phone; check the website at
http://www.ndbc.noaa.gov/station_page.php?station=FWYF1
I would also recommend getting the Bimini Cruising Guide, it is a Vantage Point Guide you can find at Amazon. It has lots of valuable information. We went in August, the Gulf Stream is traditionally smoother in July and August, so I would plan my trip during one of these months. On the way over the water was very calm and we ran around 32 MPH until we trolled for fish; the return trip found a few 3-5' swells that slowed us to the mid-twenties.
I would suggest practicing with your boat in various conditions; pick a windy day and get out in some rough waters to familiarize yourself with how your boat handles; the middle of the Gulf Stream is not the place to stuff a wave for the first time. Know yourself and your boat; know the limitations of both! Most importantly, HAVE FUN!!!!