1997 Nissan Altima About 170000 Miles On Recently Been Having Problems First

i have a 1997 nissan altima with about 170000 miles on it and recently have been having some problems. First my oil started leaking from the front of the engine on the side near the altinator i think(it has a belt and a red top on it) it stopped leaking as much oil after adding several sedatives for seal repair (it still leaks just not smoking). Recently i tried to drive my car no more than 5 miles down the street the inside if the car stated to smoke, I pulled over and decided to go back home. Home was up hill my car then started smoking from the back, all through my trunk, i pulled over let it rest and have not gone too far in my car nor up any hills since. Some one told me that 1. it could be a blown head gasket 2. the front seal needs to be replaced. I really need to know whats wrong with it. It hasnt smoked from the back or trunk area since. it does however when i turn on the heater in my car. I checked the oil in the car recently and it was a dark brown color almost black. My car starts fine the engine and car shakes when the car is idle. i also have a check engine light on but ive had that checked its the knock sensor.