purchase of 5.1 guntha bagayat land

Hi I plan to buy 5.1 guntha of bagayat land from the owner. Is this the minimum qty of land that I need to purchase to have my name on the 7/12 extract. Also there r two poultry sheds on the earmarked land. Can I use both the sheds assessment utara to build a residential house. Pls advise Thanks Devyani