Hey Vinod,
First bear in mind this is primarily a UK based forum so any answers you get will be strongly skewed in that direction.
1. Yes, you can do a PhD whenever you like. I did mine after several years in industry and an MSc, it's not an issue.
2. The point of a PhD (generally) is that you pick what interests you and then go and research it, so pick what you want to specialise in and go for it.
3. 3-4 years
A special point that regularly comes up is financing. As you would be a foreign student there would be very little chance of obtaining a fully funded post. Foreign student fees are horrific, so be advised and come prepared to spend aboouuut £17,000 - £28,000 per annum, depending on the university.
Hope that helps.