1991 Nissan 200SX Why does white smoke come out from the ex

I have got this Nissan 200sx 91 and when I turn it on when its cold no smoke comes out until very long time, but when I drive it for a few minutes etc and then stop at a traffic for about 2 3minutes and sometimes more, white smoke starts coming out of the exhaust pipe.

I was told it could be the exhaust valve or piston leak, turbo problem, head gasket problem, or valve stamp seals need changing. I really need to know whats happening or what I should tell my mechanic, because the last one i went to want me to pay him up to 2 grands or so just to fix the leak that he just thinks is the problem.

THanks