4-8 oz of cocoa is about the right range for me (I like making a paste with hot water and adding it after fermentation ends).
Your plan sounds fine, although I might go with a bigger/richer base beer rather than building on an Irish stout kit. You can always add lactose at bottling to taste. If you are doing extract with steeping grains I'd skip the oatmeal, it has starches so it really needs to be mashed.
I did a mint chocolate stout a few years ago, but we used dried peppermint. It worked well I thought, but it depends on what sort of mint flavor you want.
Hope that helps, and welcome to the site.