Letters

Yes. Letters is free to play on Steam, so there is nothing to buy to start playing.

The minimum requirements for Letters are: Windows 10, Intel HD Graphics 5500, Intel Core 2 Duo, 1 GB RAM, 300 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Letters is a single-player game.

Mostly, yes. Valve rates Letters as Playable on the Steam Deck: it runs, but may need some manual setup.

Yes. Letters is the full base game. You need nothing else to play it.