All very good questions. You seem to have a good feel what info. you will need for to keep this little guy happy. It is always best to ask these questions first and have everything set up ahead of time. This avoids any undo stress for you and your turtle. Just for future reference on any additional animals you may want to have. Now on to the important matter answers to your questions!!
1. Do they spend the majority of their time in water or on land?
They will spend a good part of the day basking on land and then dipping into the water for feeding and regulate their body temperature. They are aquatic so you only need a basking platform for him and them water in your tank for him to swim in.
If he is a hatchling you can start him in a ten or twenty gallon tank but will have to step up to something larger as he grows. You can get a floating platform for him at any petshop or even check some of the pet supply places on line. They have several types that attach to the side of the tank with suction cups and float on the surface of the water.
2. Other than lights do they need a source of heat?
Yes you will need a heat source for the basking area the air temp should be 86 degrees or so and a UVB light for the basking area as well. Also a heater in the water if it does not keep temperatures night temps of 72-75 degrees.
3. Recommended size of cage for full grown turtle?
Well 55 gallons is even a small tank. I would say 75 gallons would be suitable for an adult.
4. Are there any concerns in making an outdoor pond for the turtle? Animals? Depth? Etc?
You should make it 3 feet deep at the deepest point and make sure it is fenced off so that the turtle cannot escape. General with sort of depth they can escape predators but if it is a small pond you would really try and design it so that nothing can get in the area to eat him (like raccoons). Also I would not consider leaving him out full time until he is at least 3-4 inches.
5. Is it better to leave turtles out in the winter or bring them inside (I'm in NC)?
It is your call. It first depends upon the species and if they capable of hibernation in your area. If this is a painted NC temps would be okay for it BUT again NOT until he is a bit more mature. Just to be on the safe side. Realistically-- if you have one turtle it would better to just bring it in and not have to worry whether or not something is going to happen to it while it sits on the bottom of your pond hibernating. Also you need winterizing equipment (like an ice chaser or a bubble system to keep a hole open in the ice all Winter). Again -- it is easier and safer to bring a single animal in once water temps stay consistently below the mid sixties.
6. About how much does a turtle eat?
If he is a hatchling you can feed him every day once a day an amount of food about the size of his head.
As he gets to be 2.5-3 inches you can feed him every other day.
Hope this helps you out and good luck with your new little friend.
Oh you will also need filtration for the tank too. What size tank will you be keeping him in to start off?