You say you are on a budget, but there is no real clarification what that budget is. That said, I will assume that the budget is tight, and you are looking for the best bang for the buck.

The case looks like a good option, assuming that those are 3.5 inch bays.
It should give you plenty of space for expansion in the future if you want to do that

RAM prices are pretty nuts right now, so I would definitely not got balls to the wall with 128 GB of RAM. 16 GB of RAM should be more than plenty for a NAS server. Maybe you can even get away with 8GB? I’m using 16 GB of DDR3 RAM in my NAS server (which is also running Jellyfin and Nextcloud) and it’s running fine.

Speaking of DDR3… Have you considered buying your CPU, motherboard and RAM second hand? From what I hear the prices of DDR3 RAM are not nearly as elevated as those of DDR4 and DDR5 RAM, and DDR3 is plenty sufficient for a simple NAS.

Be sure not to skimp on the power supply. Most consumer power supplies are not built for only running HDDs. I’m running a Corsair RM550x in my server, which is capable of supplying 130W on the 5V rail.

Good luck with your server build!