You can often find a bundle containing both Portal and Portal 2 for a price only slightly higher than a single game.
Unofficial versions typically cannot connect to Steam servers, making the excellent co-op mode inaccessible. portal-2-pc-game-free-download-full-version
Portal 2 is not a "free-to-play" game, but it is frequently available at a very low cost through official channels: You can often find a bundle containing both
Portal 2 is well-optimized and runs on a wide variety of hardware: Windows 7 / Vista / XP Processor: 3.0 GHz P4, Dual Core 2.0 (or higher) Memory: 2 GB RAM portal-2-pc-game-free-download-full-version