The only buffets that I'm aware of on Hilton Head are at some of the higher end hotels, like the Westin and the Holiday Inn near Coligny Plaza.
There are a lot of casual restaurants. When you get to the resort, as for the restaurant guide that has all the menus in it. It's magazine-sized, but printed on newspaper stock. This will be the best way to find out which restaurants appeal to you and what their prices are.
As far as ice cream, we always go to Hilton Head Ice Cream at least once each trip. To get there, turn right out of the resort and go towards Sea Pines Circle. At the McDonalds just before Sea Pines Circle, turn left (Orleans St, I think??). There is a strip mall ahead of you -- turn right, then kind of bear left around the stores and restaurants. It's kind of on the back side.
There's also a new ice cream place just past Sea Pines Circle, but before you actually enter Sea Pines (by the movie theatres and Harris Teeter) called Marley's. Haven't tried it yet.
Have fun!!