Whew! So much to say, so little to go on. First...you need an architect. Hire one. Then you need an engineer. Hire one of them too. Hiring a contractor to consult on cost and practical issues is a good thing too. Once your team is assembled, listen...hard! Best of luck!

One of the first issues is the orientation of the house. Solar panels do better on the south side. You should research whether your roof needs to support more weight. You will also do well to consider all aspects of solar building --- you can find books available in e-format for this. Go to Amazon and read reviews carefully. Things like roof overhang, kinds of windows, etc, can make a big difference in cooling your house (I'm assuming in India, heating isn't a big issue). For the solar panels, what's available where you are is a starting point. I will note that in sunny countries, like Turkey and Tunisia, I see a lot of solar hot water on roofs, but not so many solar panels.

To save cost ideally RCC brickwork & external plaster should get completed for upper flower I.e. 2nd & 3rd floor. and then based on budget internal finishing should get completed.

Dear Priya, If you could share a sketch of the room layout, you would get many suggestions easily. Questions like: Where is the entrance and the other doors and windows in the room have to be answered. The type of staircase and no.of steps will depend upon the height of the room and the furniture plans, etc.