I have Xbox 360. As long as you take care of them they will last long. And they extended the warranty on them just in case. The first Xbox's they released were the ones that were failing all the time so the newer ones are more sturdy.
I also have Halo 3, but I'm not sure if you will like it. It is a really game with the best campaign than any game I have. The multiplayer is also very fun, but also challenging. It will take time getting used to.
Halo 3 has a very large community, mostly because it is the main game for MLG ( Major League Gaming ). Major league gaming is mostly for the xbox and you travel to an event with a team of your's and you compete against other halo teams to get to the top. And you can win up to 100,000
Now back on topic, If you are a competitive gamer you will probably like it but there are a lot of things to learn. If you are a casual gamer it is also pretty fun when you get 3 people to team with and play games with them. You can also play customs where you can make you gametype and all in all it is a very fun game.
You don't have to just get an xbox for Halo 3. There are a lot of good games that you could also get, so don't limit yourself.
Rating- Since I am a competitve gamer I give this game an 9 out of 10.
Solely on the fact that Halo 2 was amazing and way more competitve and they made Halo 3 a bit more easier to appeal more the casual gamers. Not that big of a deal though.