I'm diggin' the AR, dude, but I have to say... tan furniture? =\
It looks very light and pointable, though. Have you checked out Magpul's stuff? Their pistol grips and stocks are certainly interesting.

Yes, .308 ARs exist, but you can't convert an AR-15 to fire .308. .308 ARs are called AR-10s, and are awesome. KAC makes them (SR-25, and like $8k) and DPMS makes them too.

An assault rifle is a full auto or select fire firearm chambered in an intermediate rifle cartridge. The pictured AR is not, since it's not full-auto or select fire and can't be readily restored to fire in bursts or f.a--if it did, it would be worth like $15k and there would be a LOT of paperwork behind it... or it would be VERY illegal. The pictured rifle is what I like to call a sports-utility rifle, because it's good for both sports and utility.