For 100 $ you can grab almost any B350 board. The question is, if you want anything extra, like a m.2 passive cooler or rgb controller on it. I'd prefer one from Asus.
As for the PC case look for deals, used parts or one of these:
Sharkoon AM5 Silent
Thermaltake Core V1 with high airflow
Corsair Carbide Series 100R with side panel window
or read this review:
https://www.gamersnexus.net/guides/2922-best-pc-cases-under-100-dollars-2017-edition