it is..................... why wouldn't it be , just install it to steam , run hl once before doing so , you will need patches aswell (if you have old installers of 1.2 you used) the current version is 1.2.3 (get the full installer) or if your'e lasy to get that , then just use the 1.2 installer and get patches 1.2.1 , 1.2.2 & 1.2.3 (install them in order , 1.2.3 is just a few map fixes , you can just install up to 1.2.2 if you want , theres no client dll changes in 1.2.3)
First, Welcome to the forums.
Yes, every HL/ HL 2 mod uses Steam to connect now.
Won was replaced with Steam a long time ago.
Visit the Steam website Here.
The lastest version os ESF is ESF 1.2.3 which can be found Here.
Hope I have helped.