For a first Colt rifle. I would recommend the 20" inch barreled M16 look alike. It is instantly recognizable and fun to shoot. A quality built gun will shoot forever. With due care, you could sell it and quite possibly recoupe your investment.
If the bug bites you, once you get the hang of it, then you can learn about all of the options/parts/pieces that you can add to it.
Once addicted....these forums are FULL of various 'holics. Magaholics, M4aholics, etc.
I know of one guy who is trying to get an upper chambered in every wildcat based on the .223!
If you fall that hard you'll be trolling for pre-bans for sure. All of a sudden new clothes are not a necessity, nor are cars; women friends suffer and home repairs get deferred. Cash hoarding for pre-ban purchase has begun.
WARNING! You are on the highway to hell!
I won't even get up to go count how many I own and the various parts for them. In my own case I'm still in control, as I'm only a recreational user. Other don't have the metal discipline I have and are hopelessly smitten. These fools are too weak to stop their compulsive acquisitions. I have more stuff than them anyway.