So far as PC gaming is concerned, the kind of titles I'd want to play are those Total War franchise entries which I've missed out on in recent years, such as Three Kingdoms and Thrones of Britannia; as well as... HBS' BattleTech.
I'm not sure how well such games would work with the dual touchpads on the Steam Deck, or how clearly they'd be viewable on the screen (in terms of font size). That said, Valve are reportedly working on an official dock, which would perhaps make keyboard and mouse usage on a TV/monitor that much easier.
Given the install space used up by these games, however, it would appear that the 256 GB model would be the minimum required to install more than one or two such games at once before being obliged to add a microSD card.