While I am aware that both games are different and people who like one, might dislike the other, to me they are inseparable. Think of na era in gaming history, when people tried to take make cool and unique stuff and wonderful games were created.
Anyway, for those unfamiliar or returning:
>where to get games?
GOG or Steam, with GOG being preferable or just sharpen you cutlass.
>which are worth getting?
The original BK (comes with expansions) and SS Gold (included Forever) is where it's at. SS2 is revered, but has the mission pack syndrome of being aimed at veterans (hard) and Resource War doubles down on that while adding some RTS mechanics and fuel tedium, included in GOG's SS2Gold.
Blitzkrieg 2 is full 3D, but not as good as the original. Destruction is great, but infantry models and animations are ass. Gameplay can be salvaged by mods.
Sudden Strike 3 is also 3D and was shit on release due to unit pathfinding, which was fixed in the expansion that also patched the base game. Still content is slim, mostly long and tedious mission with shit like infinitely re-spawning tunnel-japs.
>what about Blitzkrieg 3 and Sudden Strike 4?
BK3 is complete trash that doesn't work on many systems anymore, avoid. SS4 is a more arcadey take on Sudden Strike, think Codename Panzers? The base game sucks, but the expansions are kind of ok, just don't pay full price of the it.
>Mods?
https://www.sudden-strike-maps.de/
https://www.blitzkrieg.be/bkmods
Many recent SS mods are standalone games.
>Linux?
Yup, on Steam use older Proton and on Lutris Proton-GE works fine for both series, default wine doesn't. One thing to know is that some SS standalone mods like RWM 8.5 have the default exe being a lanucher and it won't work, you need to copy the files from game\code\release to \game and use game_exe.exe as the executable.