985
Half-Life 2 is currently 100% off for its 20th anniversary, plus a major update
(store.steampowered.com)
Bonus: it also seems that the episodes have been rolled into the base game. Full details of the anniversary update.
If you're gonna start with 1, I recommend playing Black Mesa as the updated graphics and qol stuff make it easier to tolerate in modern times.
But that's just me. There is probably some crucial reason why Black Mesa is not the definitive version, and I'm about to hear why
I’m with you, but probably one good reason to consider Black Mesa “wrong” is the reimagination of Xen. Even though Xen is the weakest part of the original and BM did an excellent job at it.