Fucking legends.
So many great games and mods came out of this… Counterstrike, Day of Defeat, Sven Coop, They Hunger. The amount of hours of gameplay for the price of a single game is unbeatable. Many people started their careers as Half-Life modders.
I believe you are right as well, my friend had the game new and I think the SDK was there, but not TFC.
The soundtrack for Quake was by Nine Inch Nails too. Probably one of the best game soundtracks still I would say.