Ingredient, what exactly is Red chili powder

The Korean version is made simply of ground red Korean chili peppers- and for this recipe you'd want the sort more finely ground than flakes, but not quite powder. However, in a pinch you can substitute cayenne, Allepo pepper, or any other straight red chili flakes/powder. What we call chili powder is a mixture of red chili and other spices, which will change your flavor profile more to the Mexican style than Korean. I'd go with the Allepo pepper flakes myself, if I didn't have the gochugaru. I'm like Half-Pint and don't find most cayenne pepper has a lot of taste.

No, I don't think you want to use "chili powder" because it is a mixture of chilies and other spices/herbs (like garlic). Cayenne might be better but I find it hotter than gochugaru, Korean red chili flakes/powder. And it is less fragrant (actually, I find that most cayenne has no flavor/fragrance, only heat).