Buy the Orange Box, that will have games: Half Life 2, Episodes One and Two,
I would then buy Half Life on its own. That way you can play them all
chronologically. Plus you will get Team Fortress2 and Portal, which is like
a 3d lemmings. Of course, that's just looking at the steam shop, if you get
the games from an online or high street shop you could get the orange box
and have hard copy instead of doing a giant download. I would not recommend
buying any of these games second hand from the likes of Play Trade. Go here
for starters: www.steampowered.com you will need to download and register
with Steam - click the GET STEAM NOW button. you can then buy your games
online, or get box product from the shops and it will install into steam.
Make sure you have a lot of space on whichever drive you install steam into.
After you have downloaded/ installed all your games, you can back them all
up onto DVD through the file menu, so if you ever reinstall you download the
steam client again and can restore all your games without having to wait
hours for them to download again.

On the SP2 you have
   HL "normal" singleplayer
   Coop in split screen
   And Decay introducing Gina and Corlette

Gamespot carries articles, reviews, videos, interviews related to the PS2
version
http://www.gamespot.com/ps2/action/halflife/index.html

A small fragment and interesting info from one of the interviews:
Remember the locker where Gordon get his HEV suit?
Two lockers are empty
It's actually HEV suits belonging to Dr Gina Cross and Dr Corlette Green
At the point where Gordon enters the locker Gina and Corlette have already
collected their suits during decay ...

See the interview here:
http://www.gamespot.com/video/466377/2797748/half-life-movie-4

- Peter