If you want to try the substitution, this is how you do it.
I use the USDA nutrient database for this type of substitution, it is very helpful: http://ndb.nal.usda.gov/
The key things that you need to equalize are fat and water.
One cup of sour cream has 171g water and 20g of fat, while one cup of buttermilk has 221g water and 2g of fat.
So, if you use one cup of sour cream in the recipe, you will need to reduce the oil by 18g and then add an additional 50g of water to correct the fat and water content of the sour cream.
This recipe does not have enough leavening to switch it to baking soda and have all the sour cream be neutralized, so if it were me, I would probably stir 1/2 teaspoon of baking soda into the sour cream and allow it to sit long enough so that the baking soda has a little time to react with the sour cream. That way you can neutralize the pH without over-leavening the recipe. Leave the baking powder as is.
All that said, I’m not sure how the coffee will taste in a recipe with sour cream- maybe great, since the sour cream will be neutralized, but maybe not, since the coffee will bring some acidity and bitterness. You might consider baking up a small test batch to check the flavor.