Alphadeck

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

The minimum requirements for Alphadeck are: Windows 7, GeForce 7200 GS, Intel Core 2 Duo, 2 GB RAM, 500 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Alphadeck is a single-player game.

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

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