RE: maui rental cars
It has been a while since my last trip to Maui, but it's my past experience that you can rent almost any type of vehicle. The least expensive ones are very small and require you to use racks of some kind to haul your gear around. I've found that the standard car rental companies normally don't offer racks for rent, so it's best to bring your own soft racks.
If you anticipate driving a lot to sail, aj's recommendation is probably the best choice, as vans (or possibly station wagens) make the loading, unloading, storage and security concerns so much easier overall. However, going the van route isn't the cheapest, either from a rental cost or gas consumption standpoint.
Another thing to think about is the size of your party or family. If it's just you, that makes things a lot simpler, because you can load many smaller items (sails, booms, etc.) in the passenger compartment.
Lastly, there's one more thing to think about. Are you going to take your own gear or rent? In all but my first trip to Maui, I always brought my own gear. When you bring your own gear, you usually have a much greater volume of stuff to haul around, and the convenience of van makes the most sense. If you rent gear, the quantity of stuff is usually far less, and you can be more flexible and go with a small car.