Inflatable boats tend to be much less expensive than any other sort of boat. Rigid boats may perform better than folding or inflatable boats. Folding and inflatable boats have the advantage of easier portability and storage. If you plan to travel with your boat, a folding or inflatable boat will be easier to get on airliners. You can pad any boat, but it should fit you fairly well to begin with.
Some boats fit big people better, some are better for small folks.
The size of your feet is a consideration too. In general, a sea kayak needs to be comfortable because you are going to be in it all day, perhaps without a break. Some people prefer a looser fit in a sea kayak than in a whitewater boat, allowing space to stretch and move about. Another thing to consider is cockpit size. A larger cockpit can make it easier for a person to enter and exit a boat.