Where is a reliable source for finding the average maximum mileage in
the life of a car?   For example, how many miles should the average 
2000 Nissan Pathfinder go in its life?  100,000?  200,000?  Would like
a website that has this info on many cars.

With proper care, fluid changes per the manufactures schedule or
sooner, any car or truck engine should be able to last 200,000 miles.

My 2001 Subaru Outback is at 142,000 miles right now. Have you checked
to see if there is a Nissan "high mileage" club?

I have my doubts about this information being defined anywhere... you
would have to clearly state what repairs/maintenance is reasonable to
keep a car running in its old age and then have thousands of cars
stick to this strict maintenance schedule and record the results.
For a small record, my parents bought an 83 Tercel new and then I took
ownership when I was 16.  I tried to kill off the car but it just
wouldn't die.  I finally gave up and bought a new 2000 car when the
Tercel had 287,000 miles... never broke down once.