Not speaking from a great deal of experience here, just reflecting on my own experiences... the 12" will show you a lot of dim things, and the GSO scopes do seem to be a good way to go.
The only thing I'd warn you about is that the 12" is physically quite large. I have an 8" GSO dob and it's quite big - you don't really appreciate the size until you grab it with both hands (I knew the dimensions before I got it, but I didn't appreciate the volume
. Given that you live under dark skies, transportation probably isn't a huge issue for you, but moving it from inside to outside might be enough of a pain to make you decide to stay indoors on those nights where you're not really fired up. You can still see a lot through an 8" or a 10", but if size doesn't matter to you, obviously you'll still see more through a 12". I have been extremely happy and impressed with what I can see with my 8".
On the eyepieces - the ones I got with my scope from Bintel were fine. I have three plossls - a 9mm, a 15mm and a 2" 28mm. What you are looking for is ease of viewing and clarity of images. Stars should be dots (not fuzzy) and it should be easy to see stuff when you look through the eyepiece. A lot of cheaper eyepieces are hard to look through because your eye has to be in just the right spot to catch the light. With better eyepieces it's less straining to see and you'll see more.
I'd get the scope first, check the eyepieces out over a few nights of viewing and then decide what more you need. Personally, I've decided to get a Barlow because even with the 9mm eyepiece Mars is only a small disc (in my scope the 9mm eyepiece gives me around 130x I think).
I also got a laser collimator. I'd get some kind of collimator right off the bat because you'll probably want to check the scope is collimated when it arrives, and if you suspect it isn't you don't want to wait a few days for a collimating tool - I've heard the 12" models go out of collimation pretty readily (ie: the two mirrors inside the scope become slightly mis-aligned).