First, have the basic idea of what you expect from the agricultural land that you intend to buy. The returns you anticipate and the crops, the time horizon for realizing your dreams. Is it just a means of investment, additional income or is it going to be the sole source of income?
For rice and ragi(finger millet) South India states of TamilNadu, Andhra and parts of Karnataka are good. For spices and coffee crops, Chikmagalur, Sakleshpura is good, for arecanut farming the best states are Karnataka and Kerala. For fruits crops like Mango, UP and AP account for 25% of India's production, Karnataka and Kerala follow. Bananas need deep rich loamy soil with good drainage, TamilNadu, Gujarat, Maharastra are among the top 10 states in India.
There are many agricultural land consultants available in India who would give your professional advice as to why and how and where to buy.(agro corp global- consultants for organic farming) (Hosachiguru - agricultural asset management company). I mention this because, you need to know about the soil base, the water table and the levels for the near future, the planned developments that are coming up, the proximity to irrigation canals, government or forest lands and lastly the people to facilitate the numerous steps in buying and registering the land.
You need to check the legislation of each state. For instance, in Karnataka, you need have an agricultural background and need a 'Pani' and if your annual income is more than 25 lakhs and you are nonagricultural (by family profession), you cannot buy agricultural land. So, you need to be very careful. Also, you need to be cautious about buying large tracts of land in interior places because these would have many families and their members who have a stake in it, some would be illegally held or held in benami names and land mafia will be in the fringes. So before purchasing please research, seek a good reputed lawyer who can verify the mother deed etc.
Some experts suggest advertising our intention of buying a particular land in the local/regional papers soliciting any potential disputes from connected heirs or stakeholders.