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Chris
It's important top take your time, it's your hard earned cash. One of the best ideas is to get along to an observing or club night or a Star Party and see the various scopes. As long as you are polite most observers are happy to let you look through the scopes.
An alt az mount is one that simply turns and goes up and down while an equatorial mount is one that has an axis aligned to the polar axis of the earth. For astro photography you need to keep the same field in the view without rotation for long periods of time. DSO's can involved hours of exposures as they are very faint. If you try and photograph through a scope mounted on an alt az mount, the field will turn giving you streaks rather than pinpoint stars. Because the Field of View rotates at the same rate as the earth turns with an equatorial mount, this amkes them suitable for imaging. (hope I have explained this OK, )
A dob is simply a variation of alt az mount, and so is not suitable for imaging except for short exposures (moon and bright planet). Of course the scope can be taken of the dob mount and put on an equatorial mount.
Yes there are dobs with go to and yes they can be used in manual mode. IMHO if I was going to go for a go to, I would choose one of these rather than a equatorial mount as it eliminates the need for polar alignment and still gives large aperture for the $$$.
Disadvantages of a dob. Probably the main one vis a vis a small scope like the 8SE is bulk. If you go solid tube the are big units and you need a bit of physical work to carry them. Of course there are collapsible and truss tube models that elliminate much of that issue.
I have a 12" solid tube dob and if I didn't have a wagon, I would struggle to transport it to star parties etc.
One other possible disadvantage against the 8SE is you have to learn to collimate. Thats certainly not hard to do and after the first 3/4 times you can do it in a couple oif minutes.
Hope all this helps
Malcolm
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