How old is not "too old"?

I would not recommend buying an old car even at "bargain" price; no matter how many new parts you put into it, it will be still an old car that will need more and more repair later. Simply scratch out from your list anything older than ten - twelve years. Perfect choice will be three - five years old car because the price is greatly reduced and you will still have few trouble-free years.
One important detail: 2000 is a model year, not the production year. The car produced in august 1999 may carry 2000 model year as well as the one produced in July of 2000. This is related to all cars and model years. To know exactly, look for the first registration date in the car history report.