1) Yes
2) If you are making payment in lumpsum then consideration value of the flat in the ratio of contribution. If making payment in installments then installment value.
3) It should be clearly mentioned in the purchase deed. Share in the property and payment made by each owner should be clearly spell out in the deed. Payment should be made from respective individual account as per their share in the property.
4) Before executing the deal, prepare agreement to sell how to settle the entire process. Also check whether the property is mutated in the seller and any encumbrance on the property. Get the report from the sub-registrar office.
5) File Form 26QB within 30 days from the end of the month in which payment is made.
6) Also confirm whether the seller is resident or non-resident.In case of non-resident, tax implications will be different.