You need a new barrel and bolt. Have you considered a 264 LBC? Midway has some nice barrels. Mags are nice to have but you can get buy with a gi mag but understand there most likely will be some hiccups.
I made the assumption you were going to pull apart the upper you already had. If thats not the case there are a lot of options. yes you will need a gas block, barrel, upper receiver, bolt carrier, bolt, muzzle break, gas tube, some sort of rail or fore arm, forward assist. Needless to say few parts barrel is the hardest thing to come buy alexander arms, lbc custom, midway all sell them there are a few others and also a 6.5 grendal fourm.