Your network flow is from your server, to your router, to your android phone, to your router, to your chromecast. If that’s all wifi, then every frame crosses the air 4 times, and you’re doing transcoding on the phone in the middle.

Casting sucks.

I recommend 3.5s - they’ll be a little cheaper and a little more readily available. It’s also worth noting that sata drives can plug into your SAS controller, so you can use any and all 3.5” drives that you can find :)

You’ll want to learn the difference between SAS and SATA connectors. You can very probably use either. 3.5 inch is the “standard” size, while 2.5 inch was more popular for laptops. However, in the interest of density, servers started accepting 2.5 inch drives to fit more drives per rack.

You can get great deals on used sas drives on ebay, but if you don’t know how to monitor s.m.a.r.t. data / rebuild a zfs array, that can be very very risky. You need to be able to survive concurrent disk failures.

Honestly your best bet is brand new western digital or seagate drives. Buy them on amazon, but double check that the seller is legit. That’s it.