A-TACs chest rig is already in the works since I hate PC's. I was thinking about the Phalanx Type 2 Didn't know what generation helmet I should get. However, I am burning through cash fast and replicas are OK, so if you know of an appropriate helmet to make an A-TAC's loadout with, I'm all ears (I have nothing else to say).
A MICH 2000 or MICH 2001 should be fine. Just make sure to paint it in the ATACS pattern like you did with your Masada. However, I think you should be able to do virtually anything helmet wise. Seeing as ATACS hasn't been used by military yet, there is no pictures that can be used for reference to help build your kit.
There are the pictures that ATACS officials have taken, but that's another story because it is from one group and one group only. I for one, think a pro-tec half shell would look very nice painted in ATACS or an Ops-Core BUMP or FAST Helmet.
Honestly, I would just get a cheapo replica of what you want. I have a $15 ABS thin wall PAGST and with a cover on it you cant tell the difference. You can easily replace the pads/chin strap. That way you save some cash and your helmet doesn't weigh 10 lbs.