[S8track16]

To answer your questions:

1) You don't necessarily need to have a different head unit but if you decide to run stock, you will need a hi/lo adapter (I think that's what it's called) in order to give you the RCA outputs for the amp/amps. Even if you get a cheaper head unit, it will be more convenient for the future.

2) Yes, it will fit. You will need a single din adapter kit you can get from many automotive stores (I got mine from Canadian Tire). Also, Pioneer is a great brand.

3) The adapter kit will be required as mentioned above.

As for the harness, there are 2 main options. The first, there are cheap wiring harnesses for less than $10 that will work but will require you to "tap" into the ignition wire so the head unit will turn on when the car is on. Or option 2, (This is what I did) Get the more expensive harness (around $50) and just match all the wires up including the power wire and it also allows you to keep the factory chimes.