Digging a borewell in dry land

I want to dig a borewell in three acre dryland near Sengudi.

I am new to this site and from Tamil Nadu Ramanathapuram Dist. R.S.Mangalam.

We have a three acre dryland near Sengudi. I would like to dig a borewell in coming summer. Before that I need to know:

1. Maximum water yield available in our land
2. Percentage of salinity
3. How depth in feet we need to drill for good water yield
4. Cost estimate

Also, I heard from some of the farmers that only till 250ft the water salinity is high and if we go more than 250 to tlll 400 ft the water is good. The thing is that we need to stop the water entering bore till 250ft using clay around the pipe, but I did not understand how this will work practically? is this practice done by anyone earlier? Also, whom should I contact for this in my district. If you know any contact number or mail ID please let me know.

Thanks & Regards,