Your mother is eligible to buy agricultural land in Karnataka as her income is less than Rs 25 lakhs per year and she is holding agricultural land in her name in TN

She can buy land in Karnataka... What ever will be the sale consideration amount , that she will have to show in white and its source .since she is working she can show as savings..

1) your mother is an agriculturist and can purchase agricultural land

2) to pay safe obtain permission from DC

3) DC should provide permission as your mother income is below 25 lakhs and she is agriculturist

Since your mother has an agricultural background, she can buy agricultural land in Karnataka.

The other formalities as per Karnataka land reforms act may be complied with as per law in vogue.

1. Is it required to apply to DC for permission before land purchase/registration?

A declaration under section 79 B of KLR Act

2. If I produce all proper papers related to the land she has at TN and her income form pension and house rent then there is any chance DC does not provide permission?

You can provide the information about your mother's agricultural background details, the authorities after verification may grant permission.

3. If DC does not provide permission, is there anyway that can be legally challenged?

First you may apply for it and then think about legal options when you are told about it by the authorities.