Tyre life will depend on driving style, types of roads driven and type of tyre you have fitted.
Having correct inflation is very important to maintain the life of your cars tyres and also maintain your cars handling.
There is a placard on your car that has the minimum recommended tyre pressures for your car.
Run flat tyres are tyres that when a flat tyre occurs they can still be driven on for a limited number of kilometres at a lower speed till you can get to a tyre shop for replacement.
If you look at your tyres you will see a tread wear indicator which show how much tread is left. These are designed to show when only 1.5mm of the tyres tread is left remaining. Tyres that have aged may also need replacing if they are showing signs of age related wear.
No, tyres vary greatly in the amount of grip they provide with the roads surface. The cheapest set of tyres will generally not perform as well as a more expensive set of brand name tyres. Cheap tyres will take longer to brake your car to a stop in an emergency and offer less grip through corners.