Good choice for an AR. The mid-length gas system is becoming pretty popular.
I'd recommend buying lots of magazines. I have both standard USGI mags, and Magpul PMags, and the Pmags are hands down the better of the two. That's all I'm buying from now on.
Additionally, use a good gun lube, and read the following thread to learn about a good cleaning and lubrication procedure:
http://www.m4carbine.net/showthread.php?t=354902 . The gun shouldn't be dripping with oil, but there needs to be lube in certain spots. A little too much is better than not enough in the case of an AR.
As far as mastering iron sights, then getting an optic, I disagree. Yes, you need to learn irons, but red dots and/or holographic sights are very common these days, and if you intend on using one, I'd start using it right from the get-go.
I have an EOTech 512, mounted on a LaRue Tactical QD Mount. It's nice because it places the iron sights in the lower part of the EOTech's window, so that you can still use the irons if need be, but the EOTech reticle is unobstructed. Here's a pic of mine:
The QD mount also allows you to keep your head up, and be more aware of your surroundings. Situational awareness is good, to say the least. It's amazing how fast the EOTech (or whatever optic) lets you get lead downrange. One more thing, if you do get an EOTech, I'd also recommend the GG&G cover. You can see it in my picture, gotta keep the optic safe when not in use.
Also buy or make lots of ammo, and shoot the gun, and shoot it a lot. Surplus ammo works quite well for training. You can get a case of XM193 (55gr. FMJ) for around $350, less if you really search. I use Ammo to Go:
http://www.ammunitiontogo.com/index.php/cName/rifle-ammo-223-556