Gpa property

Mr x died thus his children a,b,c,d inherited his property and sold property 1 and 2 in 1986 and 1989 by way of Gpa collectively to Mrs y. This Gpa was notarised but not registered. In 1990 Mrs y built a house on property 1&2 combined and has paid towards bbmp and bescom. They have paid all taxes and power bills till date. In 2010, mrs y had transferred both properties and house to her son by way of SALE DEED and paid the necessary stamp duty. It's a B KHATA PROPERTY. Today, in 2015 can I buy this property. They holding the property for about 20-25 years becomes a factor in case of disputes?? Is it advisable to buy the property. If so or not?? Please advice as soon as possible. 1) can original land owner claim on the property. 2) can legal heirs of original land owner claim on the property. 3) do i have full rights on the property. 4) do i face any legal problems on the property in future. 5) can i sell the property. 6) does the new purchaser face any legal problems in future. please clarify and give correct suggestions. thank you in advance.