Focus LS LX 2005, Problems, Services and Costs

[Rezab]

I recently bought a used Focus LS LX 2005, it had 49575 km on it at that time and 45000 km service had been done before my purchase.
I have it for about 3 weeks and now it has about 50000 km.
The other day I noticed a scream like sound when I push the brake mostly after 20Km driving, not always but when it starts then continues (every time I push the brake after start). Obviously it's related to brakes or brake pads.
But also there is a vooooo sound (I guess from the rear of the car) that starts after 20-30 Km driving but not always and sometimes stops sometimes continues and goes higher. This one I'm not sure about its cause!
Then I booked a service time by Denmac Ford - (Indooroopilly) cause I'm in Auchenflower and this is the nearest ford dealer base on ford web site.
I'm not sure this problems (and some other minor problems) are covered by warranty (the car has 4 month yet) or not so would you guys advise me in this regard? Also how much it will be cost approximately? Actually I'm a visitor here in Australia and I can't believe that you have to pay 300$ or something like that just for simple service like 15000km service!

In addition to those weird sounds, here is the problems (or may be they are usual and I don't know) that I apreciate if you take a look and let me know what you think:
1- it doesn't start (ignite) smoothly, I have to hold the ignition key in position for 2-3 seconds.
2- according to trip computer it consumes 11 liter per 100km! isn't it too much?!
3- rear driving light doesn't work.

And finally, I need to replace the rear tyres (one potenza ,one goodyear! yes the last owner mixed them) the front tyres are Turanza ER300, what do you suggest?
1-buying same as front tyre and use them on front and put fronts to rear? (expensive)
2-Buying cheaper tyres and useing them at rear?

long post! I know, sorry and thank you for your time and advise.

cheers