Matt
Congratulations on successful first light. M42 should reveal lots of detail. It is the brightest closest nebula to us and has lots to explore. Don't expect to see colour though!
The Tarantula Nebula in the LMC is a nice target for your scope as well so give that a try.
The accessories you have listed may be of some benefit. I will tackle them in order. The piggyback mount is for mounting a camera oon the back of a scope and using the camera lense to take pics of the night sky, usually very widefield unless you have a very long focal length length lense (300mm or longer). Can be used with prime lenses down to 50mm, but need to be fairly fast. The one you have picked is specific to various Celestron SCTs, so may not be compatible with other scopes.
If you are interested in piggyback imaging, get this book
http://www.bintel.com.au/Accessories...oductview.aspx it covers it in quite some detail.
The mount is a heavier duty version of your existing mount. If you where thinking of mounting a 12" Dob OTA on it, it will probably struggle. I note the Celestron website doresn't give it a rating for payload capacity, but from memory it is fairly low. If only looking for GOTO and visual, much cheaper and simpler to get a GOTO dob IMHO.
With the Eyepiece and filter kit, what eyepieces do you have already? The ones in the kit are basic Plossl EPs, in 15mm and 9mm focal lengths. I think you would already have a 20mm and 10mm, so the 9mm will be duplicating the 10mm. Filters are OK, but they are only planetary so have limited use. I have a full set of coloured filters and a UHC Nebula and an OIII PN Filter and the one I use most is the OIII (used for detecting planetary nebula). The coloured ones rarely get taken out.
Anyway, that my views on those items. Hope I don't sound to negative BUT I have been down the exact same path you are treading and have encountered the same pitfalls so hopefully I can be of help.
If you are keen on visual observing, IMHO the dob is the way to go. Others see SCTs or refractors as a better choice, but I can only pass on my experience.
If you are thinking of getting into imaging be aware there are few if any scopes that will be good for both. Yes you can do some imaging with a Dob, and yes you can put an eyepiece in an astrograph, but for best results the 2 areas are pretty much seperate, and serious imaging is a VERY $$$ intensive pursuit.
Malcolm
Welcome to amateure astromomy ,Matt . 
Matt to the black vortex of astronomy, loosen your purse strings and get ready for the ride. I've been back into it for nearly two years and already have a part built Observatory and a lot of expensive gear. And I still haven't taken a decent pic yet !!!
