i wanted to know how long do 1989 naissan maximas ...

If it were me, I wouldn't.  At 300k it could last another 5 years or it could die tomorrow.  Try to find something with a little less mileage if possible.

i agree with the last post.....unless you can see exactly what maintenance has been done on the car its not a good idea.....although if the car has been maintained and has had every oil change, tranny flush, tune up etc it could probably go another 100,000...rough idling, lifter knocking, or anything out of the ordinary would entice me not to buy the car.