I always wanted the M-4 (CAR-15). I just took a trip to Oregon to pick up the last of the parts for my M-4gery (none to be found in Kalifornia). I put it together last week, and we shoot it this weekend.
I was able to find a Charles Daly Defense lower for less than $200, RRA parts kit for $90, a RRA mid-length flat top upper (complete) for $799, and a DPMS 6pos stock for about $130. The rear sight was about $130 as well. A little expensive, but I think it's going to be worth it, and comparable guns are going for $1200 to $1500 around here anyway.
You want to increase your collection, this would be my choice in your shoes. For one, it's totally different than anything you have. For another, after you spend the $1000 on the gun, you have a perfect excuse to put a little more into it, thus the increase in value. Last but not least, if you don't like it, assuming you could even get the parts for it, you could always get more than what you have in it, back. In today’s market, the damn things don't last an hour on the shelf at any price. I think the hunting of the parts, and the build, were as much fun as the gun will be. Have fun!