Welcome to the forum Lauren :)
youll find the members on here are a friendly and helpful bunch :)
if you want a decent spec then you'd be better off going for an S Line model. You'll get more features and I believe cruise control is standard on the S Line.
Normally satnav is extra or Is found with a technology pack. You can tell if an a3 has sat nav when scrolling through the multi media system. If it hasn't got sat nav then it will be blanked out.
Sensors are often extra too but you'll find if you find a a3 with genuine fitted sensors then they will come up on the screen. As in when your reversing a picture of a a3 will appear from a birds eye view and will show you how close your getting to any object.
What engine are you after?
If it's diesel I'd avoid the 1.6 as I found it under powered
hope iv helped :)
cheers
steve