I would back off on the 15% abv. Maybe go for something a little easier to achieve, like 8 or 9%. It's still tricky to get a beer to that level, but definitely much, much easier that 15%. It can be done, for sure, but there are plenty of great homebrewers with a lot of experience who couldn't pull off a 15% beer that tasted good, anyway.
For the coffee, a great method is just to cold brew the grounds for a few weeks and then add to the bottling bucket. You can also soak in vodka, but cold brew water is fine too. just boil the water and let it cool first, then add the grounds. chocolate can be soaked in vodka and then added into the bottling bucket as well. that way, you can taste the flavors and add more as you see fit, rather than going in blind.
oats are a great bet for stouts if you want that creamy, thick mouthfeel. it seems to do the trick, but I would definitely suggest a full volume boil. Adding water afterwards, in my experience, gives a thin taste to the beer, no matter how thick you try to make it. for 5 gallons, add a pound or two.
The dry irish flavor has mostly to do with the yeast you use. Irish ale yeast is great for that. For a different taste that's more American, but will still yield a dry flavor, you can use California Ale yeast. If you can find it, White Labs also does a Super San Diego Yeast which has the same flavor profile as the california yeast, but it more suited to big beers like the one you are talking about.
Overall, a beer this big takes a lot of work and care, and if it's going to be drinkable at all, I think some experience is definitely necessary. I would say that you should work your way up to 15% abv. Start with 7-8, then on to 9-10 and so one. And try some kits using those flavor extraction methods.
Whatever you do, Godspeed!