The $100/day rule is a very good way to set a goal. And, to answer your second question, the Fantasy is a great ship.
However, it has been my experience that the amount of fun I've had on my six cruises (twice on the Fantasy) had nothing to do with the ship or the amount of money I spent. Cruise fun is what you make for yourself by meeting people, getting involved with activities or just relaxing and people watching or just doing what you like to do. The Fantasy isn't the newest ship in the fleet, but for the days you are on it, it can be the funnest!!
ps...it's always a good feeling to come home with enough money left over to start planning your next cruise!!