My questions are
1) Can step mother claim in this property now?
As per law a second marriage during the subsistence of existing marriage is null and void hence the wife will not be able to command the status of wife when the wife of previous marriage of her husband is active and subsisting. Therefore the step mother or the so called second wife is not having any rights or share in her husband's property even if he died intestate.
2) Can partition will be permitted EXCLUDING our deal ?
If she cannot claim a share at all then where is the question of partition of any property including the property under deal with you.
3) What should be our points to tackle the questions raised by sellers step mother?
Seller's step mother cannot raise any question with you. It is a matter between yo and the vendor, so let him deal with her about it separately.
4) Can they challenge the validation of documents & seller’s name registration of overall property after 25 years?
There is no question of validation of document. Mere transfer of revenue records will not confer title to the vendor. He should have a registered marketable title deed on his name if he intends to sell or dispose the property in any manner. If things have not been properly as per law, then there are possibilities for litigation in future on the entire properties.
5) Are we safe in this deal?
It is better to cancel the sale agreement and get your money refunded because it appears that the step mother may create problem even after you get the property registered on your name.