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A few answers to your questions:
A 6" can typically detect Quasar 3C 273 at 13.7 magnitude at the limit of its capability. Pluto is in approx. the same magnitude and can be detected.
The same objects are relatively 'easy' in a 10" which can typically detect 15 magnitude.
However the sky conditions play a big part as does the magnification used, the quality of the instrument and the experience of the observer.
Exceptional astrophotos can be made with an 80mm refractor so an 8" or 10" Newtonian can give some nice photos but a very stable mount is required. If astrophotography is your goal, put your money towards a bigger (and expensive) mount and use an ED80 refractor or perhaps a 6" f5 Newt to keep the tube weight down.
A non-driven Dob is not really suitable for astrophotgraphy unless you are doing low power video capture of the Moon for example.
Yes you can add an Argo Navis for navigation but this is like adding a GPS to your car. A nice accesory but you can get from A-B with a Gregorys map (see also below). Get your self Sky Atlas 2000 Version 2 Deluxe plus the Sky Atlas Companion that lists all the objects and their characteristics (a must) and you will never look back. Then study the atlas on cloudy nights and learn the different parts of the sky. This will prepare you for clear skies. Use Stellarium desktop planetarium too to show you what is supposed to be up at a given hour.
You need to learn how to star hop which is like learning how to get from point A to point B on your street map. Scale is the other thing you need to learn, ie. when looking at a map, just how much sky is covered by a particular part of the map.
I would like to add that out of all the telescope computers out there, the Argo Navis is the one to get since its database is very extensive and you can use it concurrently with your star atlas since it has map number references for most popular start atlasses produced. It can also show you only objects visible in your telescope so that you don't end up trying to find a galaxy for example in Sky Atlas that may be magnitude 15. For the record I don't have or want a GPS in my car but may get an Argo for my 'scope one day.
One idea for an observing session is to pick a page in your atlas (obviously it has to be up that evening) and see how many objects you can find on the map. Use the Sky Atlas Companion to see if any object is too faint and move onto the next one. This keeps you observing in one particular part of the sky and you don't end up looking at the usual favourites all the time.
Whatever you choose, enjoy!
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