You get deathmatch no matter how you buy HL2. Once you get your game validated by steam, it will download all the updates including deathmatch.
I went the Steam route. With cable internet, once I paid for the game it just downloaded. If I get a new computer, or want to play on a different computer, I just install steam, and HL2 is now downloaded onto that computer. Another bonus is getting the entire Valve back cataloge for free with a Steam purchase. I had never played Opposing Force or Condition Zero, so that was a great bonus.
If you are worried about downloading the game over and over again, you can also backup all your files to CDs or DVD once you download the first time.
Oh, and ANOTHER bonus, you get DOD: Source free with Silver and Gold steam purchases. Retail HL2 owners will need to buy DOD seperately if they want it.
Edit: Since its newegg selling the import version, that makes me feel a lot better that its not a copy.