Syam,
this is all rather spooky cos I also played the guitar for around 11 years before I got into keyboards/synth.

In answer to your question, I would strongly advise you to go for the controller keyboard route.

Why? Well, first of all it would be a more economical route. Not only are sound modules/racks cheaper than their full size keyboard equivalents, but with the pace of today's technology I think it's safe to say sound modules will soon become obsolete (as seems to have already happened with hardsamplers)!!! There are already some AMAZING software packages out right now which are well worth checking out!

If you go down the full size hard keyboard route, then no matter how much you love it now there's a chance (let's say) that in a few years time you might want to move onto something a little bit bigger and better. With the midi controller keyboard route, your gear can constantly expand and grow with you and you'll never be stuck with something you don't like.

For the kind of music you're making, I would seriously advise buying the best midi control keyboard you can find. If at all possible I would try and to get one with some assignable knobs are/or sliders so that I had as much control as possible over the sounds I was creating.

I would also invest as much as I could into making my computer as powerful as possible.

You said that you will be making pop music and mentioned a few types of sounds you were interested in.

Well... this is just a personal opinion and loads of people will disagree, but from what I've heard I think Roland are currently winning the battle for best string sounds and best piano sounds. I'm not sure about flute sounds (never done a head to head comparison with other brands) and for drum sounds I'd recommend using drum samples from the internet.