I would agree with Raw Kryptonite -- I would strongly caution against buying a 360 just for Halo4. I made this same mistake back in 2008 with GTA4. Eventually I found more games I was interested in (I'm 99% a PC gamer) but for the first year or so when I owned it and got tired of GTA4, it really felt like a waste.
Not sure what kind of games you typically play, but my experience with Halo4, and I would imagine that most others would agree, is that the offline campaign features a huge (and incredibly confusing, if you haven't really played the other games) story, gigantic environments, very cool level design, a great variety of weapons ... and an entirely lackluster gameplay experience. The actual gameplay is, in my opinion, completely unimaginative: it's just a standard shooter. Once you've used each weapon and killed one of each kind of bad guy in the first hour or two -- you've already experienced the sum total of the gameplay variations that Halo4 has to offer. After that it's just rinse and repeat, over and over.
Which is not to say that Halo4 is a bad game -- it's certainly not. I very much enjoyed it for what it was. I'm just cautioning you against taking all the hype too seriously. Fun game, yes; but really nothing original.
But if you're determined to get a 360, I also would suggest buying one used. I imagine you could find them pretty cheap these days. It used to be that the units with the larger hard drives were ridiculously more expensive than the lower-end models, but that may have changed. The HD thing is a Microsoft scam anyway -- my unit came with a 20GB drive which I upgraded to an 120GB laptop drive for about $60 two years ago.