To add simple storage you could go with something like this: https://www.amazon.com.au/dp/B0BQ6SHNP1?

I was blown away at the cool design (the case and cover are the heatsink, you install it with a thermal paste pad inside - how cool is that?!) and how inexpensive 1TB m2 cards are now.

If you want expandable storage, then maybe a NAS?

Husband set up one a year or so ago and it has been really cool. Easy to back up and access anything on it.

USB 3 is often more than enough if you're thinking of using regular spinning disks. I have a ThinkCentre mini with 3x USB 3 ports. I've connected 4 external disks to it, 2 of them directly and 2 via a USB 3 hub. It purrs happily without a fault. Just make sure the cables aren't easy to disconnect accidentally while it's working. E.g.:

Got an e sata port?

If you need to add more than one drive you should look at external enclosures. There are 4-drive models just over $100 and 8-drive models just over $200. You connect them with eSata, USB 3, or USB C, depending on the enclosure.