My house is about 100 years old. The door jamb is about 10" wide. I've replaced a few doors before, but all of them had standard jams. I'm not sure how to go about replacing this. I'm assuming I need to pull all of the trim & casing out, then pull the brick-molding off the door frame, then slide the frame all of the way back flush with the inside wall. Mount the frame, then re-install casing & trim. Also, i'm not sure where I stop removing the trim & casing at the top, since there is a top-light above the door. Below are a few pics.
Please help point me in the right direction.

to keep from marring the backside of your brick mold.
Once the door is out, the problem then becomes finding a new door/jamb that fits the opening not just properly but perfectly in order to get the reveals right, which may be a challenge considering how old and...unique your existing door is. If you over come that hurdle, just caulk the backside of the brick mold with sealant (sealant not caulk) and follow the manufacturers instructions for sealing the threshold and all should be well. Cheers!