Yes you can brew a good stout without coffee, and yes roasted barley will have a good coffee flavor, but that's why coffee is such a nice addition!
I work at a coffee shop and have access to an espresso machine, so I've brewed a stout with real espresso. That beer took first place in a local stout contest a few weeks ago, so I very much disagree with those of you who doubt that coffee can accentuate stout.
I'm assuming that most people don't access to free espresso like I do, and also that you probably don't want to drop another $20 or so on a beer, so the cold press method that Professor Frink suggested is the way to go. Cold press really only takes about 12-24 hours, so just prepare it the day before. Just take ground coffee, probably a darker roast, and add it to cold water, using 2 tablespoons of coffee per 8 oz of water, let it sit in the fridge for 12-24 hours. I used 16 oz of espresso in my stout and the coffee flavor was very balanced; it accentuated the other ingredients and didn't taste like "beer with coffee in it," so I'd recommend a similar amount. Just strain out the grounds and add the cold press to the wort right after the boil, or add it to the secondary. I added after the boil.
As for chocolate, I guess I always think of Youngs Double Chocolate stout, and would probably only add cocoa powder to a sweet stout, not a dry. But that's just my opinion, it might be great! Good luck!
Edit: In Mosher's Radical Brewing he recommends 4 to 8 oz of cold press coffee in a stout, and since I haven't actually brewed with it myself, I'd have to assume that you should listen to him.