Just bought a 98 2.2 Impreza L
A/T
84k miles; 1 owner

Need to know where to start with:

1. Check Engine light: Coding Knock Sensor (should I just buy one or is there something else to consider)
2. Noise from rear end. Kind of a friction/low grade roar (possibly on left side; hard to tell). Wheel bearing?
I heard they were pressed in. What to do here?
3. Airbag light is on. Suggestions?

Thanks in advance,

Mega