We rented a car in St. Maarten two years ago. We rented it from here in Canada. The rental place is about a 3 minute walk from the pier.
Several people hadn't prearranged. It appeared that you could pay more or pay less. There didn't seem to be any rhyme or reason as to the prices the rental companies were charging customers.
If you know you want a vehicle, I would prebook it so you are guaranteed a vehicle incase there are alot of people who are renting on the spot. This way, your price is also guaranteed.
I would not suggest a jeep or convertible as it can get extremely HOT especially after a day at the beach. We loved our air conditioned vehicle.