Its definitely a head gasket issue. I had a d15 in my 4 door when I got it that was doing this. Then one day right before my gsr swap I was about to do about a mile away from my house it started blowing out white smoke like dammit, and the heat went cold, and then it started to overheat on the guage. I drove it the mile home and took out the plugs and looked down the hole. water in the #1 cylinder so I knew it was time lol.

But take the head off,definitely get it decked if it isn't warped too much, and then maybe do a tune up while you are at it.
But it could go at any time now.