many decks can be build over the porch area and no need to do the labor hours in removing that slab.
I like a deck to come even with the door's threshold so the transition is nice and smooth. Does your door's threshold rise 1 1/2 inches above the porch?
If so, this is enough for a 2 x 6 to butt up against to make the deck even.
How big do you plan to have your new cedar deck? Will it be larger than the concrete slab you have now?
Dek Blocks found in the home centers are excellent foundations for your joists and beams and they will sit directly on your existing slab. Then if your deck is larger than the slab, which I recommend it being so, then you can go into the ground around the deck for independant structure other than the slab.