Check the service records. If the trouble spots have been fixed or upgraded, the StarMark might not be necessary.
If it hasn't already been done, compare the cost of a new (2003+) ECU and Instrument Cluster with the StarMark cost. Word has it that the new ECU and cluster fix a lot of problems. Several folks here have done this; and are very happy with the results.
Who was selling the 2000 G500?