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An ED80 will see a great many DSO's and other objects. The only advantage a 6" scope has over the small refractor is light gathering capacity, so it will see a little deeper, but not as much as you might expect. The big advantage of the ED80 is it has a clear optical path, i.e. no central obstruction in the form of a secondary mirror/lens and a much wider field of view. That's one of the main reasons why they suggest you get a refractor when you start out astropics. Because of the wider FoV, guiding the scope is easier and more forgiving when it comes to guiding errors. The big advantage of the SCT's in the compactness and large aperture (hence substantially greater light gathering capacity and higher resolution capabilities). The longer focal lengths aren't ideal for wide angle astropics (taking shots of extended objects such as large nebulae and globs), but you can use a focal reducer to remedy that. The longer focal lengths are ideal for small, tight targets such as PN's and planets, where you need the longer focal lengths to produce the increased image scale for such objects.
What is mandatory, though, and the others would agree here is you need a good mount. A HEQ5 is about as light as you'd want to go, but preferably look towards an EQ6 class of mount or larger. Depending on your particular setup. Phil's suggestion of an ED80 on an EQ6 is good and is pretty much the stock standard that most start off with. You can get away with using a fork mount on an EQ wedge, or with a field derotator if you want to use the scope in alt-az, but that just adds to the cost of the setup and more complexity.
Your D800 is a good camera and will take good piccies, but it's not really a camera for taking astropics as its sensitivity to Ha emissions is poor. One of the new Canon 60Da's would be more appropriate as they're designed for taking astropics, or you could get a camera modified for the purpose.
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