OP---do a couple of autocrosses. You won't get much past 50 mph, at best, you'll be in a controlled environment, and it's cheap. You won't be running it for the best time per se, but to get a better idea of how the car responds to abrupt turning and braking. In most case, the worst consequence of spinning out is driving over a few cones.
-In normal, dry conditions you should be able to take any turn you'll run into on a road within at least 10 mph of the speed limit and never even tempt the stability control to activate
-In wet conditions, the same generally applies, but will be greatly dependent on the condition/quality of your tires
-In snowy conditions you need to be very careful--as you would in any car. The mustang isn't the best in snow, so it's best to take your time and go slow. You will best served to get some dedicated snow tires.