Capital gain tax exemption for NRI?

I am an NRI. I have purchased a plot within a Municipal corporation limit for Rs. 6.5 lakhs in 1990 and spent 1 lakhs in 1991 for levelling the land (I do not have any doent to prove the levelling done). Now I want to sell it for Rs. 25 lakhs to a resident indian and utilise the money for buying a flat in the joint name of myself and my wife. (1) What is the tax implication for this transaction. (2) The balance money required for buying the flat is from other sources generated in India. How do I pool my sale proceeds and other sources into this one buy transaction. Do I have to show any doent for the source of income for the balance money. (3) I have so far not filed any income tax return as I did not have any income in India other than mutual fund and equity for which TDS is deducted. Do I have to file return for the plot sale/flat buy transaction showing the capital gain and its utilisation.