Questions about Core i5-8400

Enough, fits and is actually too much for your described activities... There is nothing else wrong with the components. If you leave it as it is, you won't have any problems with performance.

Well.
I think if you want to buy a 1080ti and you can, you should also be able to buy an i7 (or other CPU). In 1080p it doesn't make too much of a difference, but in 4k (which the 1080ti is meant for) the i5 might have problems.
Here is another link that might help you with this: thebottlenecker.com/#calculator