S14aLove

Ok, so my car is pushing 190k abused miles (from the previous dipshit of an owner), and I've sought advice from my families shop. All they can tell me is the motor is just worn out and will soon need a head gasket. That's all fine and dandy, but my car runs fine, doesn't seem to burn much, if any oil. The coolant level is stable and the correct color. The oil isn't milky. It pulls just as hard as my previous 240. Yet, the RPM's seem to run a bit low at idle, I'm getting a sound the seems like backfiring, but I'm hard of hearing so, it's hard for me to tell. I got it with a full Megan 4-2-1 headers to muffler exhaust system so, I'm not entirely sure it's not just the exhaust itself. The most annoying symptom is the cloud of smoke that leaves the exhaust sometimes when I start from a stop. The smoke is white, but it doesn't always happen and it doesn't smoke when driving or when changing gears. I've had my girlfriend follow me to confirm this. I am getting a timing chain rattle, but I think it's just the infamous guides. I was thinking maybe a bad o2 sensor and a lean a/f mixture, but I'm told the CEL would come on, if so. I was also told by the previous owner that the motor was swapped which kind makes me believe the CEL was either not hooked up, or is faulty. The motor is said to have 90k miles on it, but that is hardly believable. I'm new to the forums so, if I posted in the wrong area or anything of that sort, feel free to redirect me.
Any help would be greatly appreciated.