Suggestions for progressive upgradation of bike
I tried the same approach that you're outlining with a bike I bought (a Shogun), but I had heaps of trouble trying to upgrade components. Every time I upgraded one component it wouldn't work well with the other existing components. I tried to change the cassette but it wouldn't work with my derailleur (even my mechanic couldn't figure out why).

The other problem with upgrading is that each component bought individually will be vastly more expensive than buying a bike with better components in the first place.

So my advice is to live with your bike unchanged as long as you can, saving the money you would be spending on components and then buy a new/second hand better quality bike.