Buying a good horse at the auctions is a real lottery.
One method i used was to look for 2yo whos "Average Best Distance" was high but was still managing to win the 6f/1200mtr maidens.
Actually look for any age horses that are winning well outside their average best distance.
But that doesn't guarantee it will be a great horse.
The letters in the black boxes mean it has won at the "C" course, "D" distance, "G" going.
Good luck
Edit: Oh and this is the best guide ive managed to find....
http://www.startersorders.com/assets/SO6Instructions.pdf