1. Where's the memory? Get 2 matching sticks, unless you already have those on hand.
2. Since you're replacing the power supply that comes with the case, can you find a better case at that price point? Or maybe a cheaper case?
-and to answer your 2nd question above-
3. That's a nice PC for surfing the web, but for running OBS (esp. if you plan on gaming on the computer at the same time) you won't get very far, unless you plan on using NVENC to record with and don't plan to stream at all.