If you're only planning on running 1 amp then 8awg will be fine for both your battery cable and earth cable. You want OFC cable regardless of what thickness you go for. You will need at least 5m of battery cable, I'd recommend getting 6 to allow for proper tucking etc.
If you want to run 2 amps then you'll need at least 4awg, I would probably recommend 2awg or even 0awg if you're planning a large mono amp.
You can get a little crazy with wiring, there's cheap ways to do it and there's expensive ways to do it. Unfortunately, as with everything, you get what you pay for. There are SO many companies now selling copper coated aluminium wire that it's scandalous. You REALLY want to be using OFC wire for everything, but as with anything you'll pay for the privilege.
So...... with all of that in mind, here is what I would recommend.....
14m of 14awg (2.5mm) OFC copper wire for the speakers
6m of 8awg (8mm) OFC red copper wire for the battery positive (I would ask these guys if they could supply 6m rather than 5).
2m of 8awg (8mm) OFC black copper wire for the earth (Rear seat is a good earth location, sits approx 1-1.5m from where you will have the amp so 2m is plenty). This is really good stuff. If you're feeling flush you could get the power cable as this stuff too. It's excellent and worth the money.
5m of 18awg (0.8mm) any old crap blue copper wire for the remote