Any SATA or SAS drive should work just fine. If you need some hot swap caddies, you can buy them fairly cheap on Amazon: https://www.amazon.com/s?k=dell+t320+drive+caddy

I’ve personally used the WorkDone brand caddies, and they are perfectly usable!

edit: I’m fairly certain it doesn’t support four 3.5 AND eight 2.5… the form factor supported depends on the backplane that’s installed. Also know that the backplane physically supports double that number of hard drives… you’d just need an HBA card with two internal ports. See this list for some options: (https://forums.serverbuilds.net/t/official-recommended-sas2-hba-internal-external/4581)