"Do AR15s need stainless steel magazines?"
No, Aluminum magazines work just fine. I have been using Brownells magazines with great success here of late.
The rifles do apparently need a four rail forend with a powerful flashlight, laser designator, forward vertical handgrip, several rubber covers for the parts of the rail forend that the shooter isn't using,,,
They also seem to now require a fold down front sight, a flat top receiver with a super duper dot optic of the most expensive kind, a fold down rear sight just in case the super duper dot optic of the most expensive kind fails,,,
A huge flash suppressor or muzzle brake capable of reducing or creating the flash and blast pattern of a 155 howitzer is always appreciated and admired on the modern AR15,,,,
They also must have a sliding stock of the most recent and outrageous design AND that stock MUST be capable of storing spare batteries for the super duper most expensive dot optic and the powerful flashlight,,,,
Am I forgetting anything?? Oh yeah, the rifle must also wear a "Tactical" over the neck sling of the most complicated design that is nearly impossible to actually install on the rifle and once installed is guaranteed to hang up on any and all extra gear you may have attached to your person along with the spiffy, most modern design rifle....