Do them both at the same time - especially if you are changing spring rates etc. Why? Because the best way of setting up your car is to have the two balanced to complement each other for best performance. Anything else has the potential to be detrimental.
BTW - don't buy second hand - you're just buying into more trouble - remember, the guys selling them is getting rid of them for a reason. More to the point, you have no idea if the ratios are correct for your springs.
Suspension affects braking. Now personally, if it was my GF's car, I wouldn't be doing anything that might affect the braking abilities of her car... even if it is a XE Ford Falcon
