I have been told that the computer on my 2000 holds data on my engine's operation, including if the engine has been over-reved or over-heated, how long it has idled, total hours, etc. Is this true? If so, how can I retrieve this data?

I do not believe that this is totally true if a code is set a freeze frame scan would reveal operating conditions at that instant, if what you say is true the general public cannot access this data. The SIRS module acts as a black box for speed and braking at the point of air bag deployment, again not available to the general public.