What kind of numbers are considered "good" when testing a motor? I know the closer all the ohm readings and timing readings are (to eachother, respectively) the better, but what's a good ohm value for a 17.5? What about the rotor (gauss?), what's a good reading there?

probably anything under 0.5 Ohms?
Scorpion RS-3420 17.5T - Scorpion Power System

Also good resource here to check your resistance over time as the motor ages:
Brushless Motor Winding Resistance - Radio Control Info