The route as Lonny and Dianne suggested is probably the best. For us we generally take the FL turnpike to US27 and down to 192, then take Disney's World Drive in. Our grandsons loved the characters and experience on World Dr. Just says 'Your Here'. It also allows us to stop at Walmart or Publix to stock up before arriving.
Personally, I would not bother with the Sunpass. Just have some cash to pay the tolls. Unless your doing a lot of travelling off Disney you will only use it twice, not worth the 4.99 to save a little on tolls.
Ken
Edit added. If you arrive after 12 and before the 3pm check in time, most likely your site will be ready, if not they will directed you to a parking spot and you can take a bus in and check out the campground, pools beach etc.