Refuse to do so .take the stand at time of registration of sale deed you will pay by demand draft or banker cheque balance amount 

2) old lady is owner of flat .she will be executor my sale deed her children can be witness 

3) notarised agreement for sale is valid 

Dear Client, 

The new owner acquires the rights and interests in the property through the sale deed. A sale agreement grants the buyer the right to buy the subject property as long as a certain requirements are met. The sale agreement is executed on non-judicial stamp paper and is signed by the buyer and seller before the sale deed. A sale agreement requires the participation of two parties at minimum. One will take on the role of seller, and the other as buyer.  Although it's not required, an Agreement to Sell may be registered, and most do. The property must be registered if its worth exceeds Rs. 100. In order to register a property, one must pay stamp duty and registration fees that are determined by the property's worth. The registration also acts as proof in the event of future disputes.  

Hope this answer proves beneficial to you. 

1. Make  DD and pay her n the date of registration.

2. Only the seller is to be made party and no other person.

3. Notary is enough.

1. you can give the PDC but mention in the agreement for sale that the pdc will become payable only upon completion of obligations on the part of the seller and not prior thereto. this pdc it appears is being taken from you so that in the event you default in making payment of balance sale price, then the seller will encash your cheque and if it bounces then it will file 138 complaint against you in order to armtwist the buyer. so mention in the agreement very clearly that only upon the seller handing over the possession and registering the property in your favour, would the PDC become encashable and that the pdc is not issued to the seller towards discharge of any debt or legal liability and that it must not be misused by the seller 

2. old lady is the owner by virtue of the gift deed [i assume it is registered]. so only she is required to sign the documents. her husband and children are not required. they can sign as witnesses though

3. you can register it. but for that you will have to pay full stamp duty. so when the final sale deed is made and registered, you will only have to pay nominal duty since full duty would have been paid on the agreement for sale. 

instead of keeping a time gap, why not complete the final sale formalities at once? this will prevent any possible misuse of the pdc which the seller wants you to give. 

1. If you will handover the post dated cheque , then that can be used against you in the event of transaction failure 

- Hence, you should enter into an agreement with her that the balance amount will be paid by you at the time of registration of sale deed. 

2. If the said gift deed is registered from the office of the registrar , then she will be only party as seller , however her husband and son can be witnesses on the sale deed. 

3. A registered agreement is suggested .

1.  If you have reasons that you are certainly buying the property chosen, there is no legal infirmity in depositing the post date cheque but obtain an acknowledgment for the receipt of this post date cheque.  If you change your mode of payment at the time of registration, then yo can withdraw this post dated cheque and make the payment by DD or any other legally valid mode of payment. You may remember that  the seller cannot encash this cheque in advance i.e., before the date mentioned in the cheque, hence there is no imminent risk or danger by doing so.

2. First of all ascertain that on whose name is the title deed is there.

if the old lady has clear and marketable title to the property then she alone can execute the sale agreement and the subsequent sale deed.  

If she is too old with frail health  then it would be better that you obtain a doctor certificate to certify that she is in sound mental health and if possible you may get her children as attesting witness to this sale deed.

3. It is advisable that it be executed by a registered document

Do it on Rs 500 stamp paper 

Dear Client, 

A sub registration office may execute a sale deed on 100 rupee stamp paper. Nevertheless, the state in which the sale deed is being executed will determine the stamp duty that will be charged on it. In India, state-by-state variations exist in the stamp paper value required for the fulfilment of real estate sales agreements. The minimum amount of stamp paper needed, however, is Rs. 100 in the majority of states. When compared to a sale deed, the stamp duty that must be paid on an agreement to sell is typically smaller. Depending on the kind of transaction, the Rs 100 stamp paper has different legal significance. For sales of less than Rs 5 lakh, a stamp paper worth Rs 100 can be used. 

Hope this answer proves beneficial to you.

No doubt it is still valid to proceed.

However, if for any reason the seller is not willing to sell the property then this notarized sale agreement can be considered as a receipt for the money deposited as advance hence you may have a right to get the amount refunded as a relief and not the execution of sale deed, this is due to non registration of the sale agreement.

- Yes, for notarization Rs.100/- stamp is sufficient. 

You need to register the same