The 7/12 document is an extract from the Land Register of any district in Maharashtra, which gives complete information about a particular piece of land. It contains important details such as the survey number, area, date from which the current owner's name was registered etc.

2)A 7/12 extract is not a conclusive proof to an established title. It is only the land record for revenue purpose and it can be used to show the possession rights of the land

3) advisable that your wife name be on 7/12 extract to purchase agricultural land in her name 

4) there are no income related limits for purchaser 

5) obtain farmer family certificate 

1. As per the information mentioned in the present query, makes it clear that you have been adviced by my fellow friends on this forum, but little different opinions came to you, resulting little confusion in your mind.
2. Let me tell you that the concept giving the land to an agriculturist came just to avoid the labouring of poor farmer by rich people of earning money after purchasing the land on their name, but not engage in agricultural activities.
3. Having the name over the above mentioned certificate is mandate for sure, but as her father’s name is already there on the certificate, so she may able to purchase the land on her name also.
4. It is also not necessary for her to cultivate the same, but only her father or brother can cultivate the land, not any other person.
5. Otherwise, it will create problem for her as it would violate the principle of saving the life of the farmers by getting benefited in their hard work.

Rest, I ll try to give you some good precedents by Supreme Court, to give you more clarity on it.

Only an agriculturist can purchase agricultural land and if a person holds such land anywhere else in India, he can still be deemed an agriculturist in Maharashtra. The maximum ceiling limit for such land is 54 acres. However, to be able to buy the agricultural land, yourr wife's ancestors shall have to be farmer which they are actually. Your wife  will need to give 7/12 extract mentioning her mother's or father's name. Her relationship with them has to be established with supporting documents. This makes your wife  eligible to purchase the agricultural land. 

1. A certificate certifying that she belongs to the agricultural family will be sufficient for her to acquire agricultural land by a registered sale deed. A 7/12 certificate on her name is not necessary.

2. There is no such criteria in Maharashtra.

3. You can obtain a certificate that she belongs to agricultural family.

This is a local law by which the family members of the farmer are also considered as farmers for the purpose of buying the agricultural land and they are eligble to buy agricultural land provided they are not NRI or OCI.

Your wife may approach Tehsil of area where father  land is situated and make an application for Farmer certificate. Alongwith this application, you need to submit 7/12 and 8 "A" signed by Talathi. 

2) unable to find any judgments of SC on the issue 

i did search for your query whether a family member automatically becomes a farmer because her family members own farm or agricultural land

but i did not find any such proposition in any acts

also how can a person have a 7/12 certificate without first buying the land?

thats completely absurd

only after the person buys the agricultural land, can her name be entered in the 7/12 extract 

without buying the land, its not possible to have a 7/12 extract

i did inform that there are certain ceiling limits i.e. certain extent of agricultural land which a person can hold

so apart from that restriction, i do not see any other restriction against a person being termed an agriculturist

anyway i will do some further research and get back