Humanius

Prices of HDDs have increased in recent months due to the AI bubble

Here in the NL we have a website called Tweakers for comparing hardware prices. They only really list webstores that sell to the Netherlands, but it could help give you a decent indication of normal prices at the moment.

If I sort by price / TB, this refurbished 6TB Seagate SAS-drive for €122 seems to be one of the best deals I can find:
https://www.redshell.nl/seagate-enterprise-capacity-35-hdd-interne-harde-schijf-6-tb-7200-rpm-128-mb-35-sas/

Given that price, €134 for a refurbished 6TB Toshiba seems like a pretty decent deal. Though I would like to add that my experience with Toshibas is that they are quite loud compared to Seagate and Western Digital. So if noise is a concern it might be worth looking for those instead.

It would seem that the sweet spot for HDDs is as high as 16 to 24 TB at the moment (at least here in the Netherlands).
You can get a 24TB Seagate Barracuda for €479,- right now, which comes out to about €20 / TB.

If you specifically want a NAS drive though the best “bang for the buck” appears to be a 28TB Seagate IronWolf Pro for €688,- coming out to about €25 / TB.

Edit: Personally I run 8TB drives in my server, which are currently €209,- (€26 / TB) for a regular Seagate Barracuda, and €289 (€36 / TB) for a Seagate IronWolf Pro. Funnily enough 4TB drives would actually be better for NAS drives at €132,90 (€33 / TB) for a WD Red Plus.

There is no real clarification what that budget is, so I will assume that the budget is tight. My advise is assuming that 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 go 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 own 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 running a NAS worth’s of 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!