You basically need an external keyboard because the built in onscreen keyboard takes up about half the screen and the resolution doesn't get scaled down. And there's only one USB-C port so you need to buy a hub to extend it to multiple ports (and the charger is USB-C so a C to A hub means you can't charge while using your keyboard) or a Bluetooth keyboard.
Note that if you already have a laptop dock, you may not need to buy a separate one for the Deck.
When researching before ordering my Steam Deck last week, I looked into it and found that the Lenovo Thinkpad dock that work sent me with my work laptop is said to work great with Deck. (I just received my Deck and haven't yet tried this, though.)
