95 mercury tracer

i have a 95 mercury tracer with 189,000 miles. my heat works for about 5 min's then goes cold, with an antifreez smell and white smoke coming through my fents making my windows foggy with an oily base. the car jerks going up hill like it's about to die. i fill it with antifreez 2 times a week.

i was told by a mechanic that it could be the heater core. what could it be? and how much will it cost to fix? thank you