1. For the money you have issued can be treated either towards the total consideration or the as the earnest money as mutually agreed by you and the seller. it is necessary to prepare a receipt and write the amount and mode of payment and the total consideration, date , details of cheque if you paid in cheque and the seller has to sign it as he accepted certain amount.This can be attached to the original sale deed as part payment.
2. You should also make an agreement to sell with all details of the property and , total consideration, earnest amount paid, clauses of completion of payment, registration of the sale deed, handing over the peaceful possession etc. This is as equally good as an MOU, but since you will have sale deed registered in your name after completion of the deal ,so an agreement to sell is abetter option,
3. The agreement to sell can be prepared by an advocate, so that the time clause, and other essential clauses of performance of contract can be legally included.
3. Do not go for a power of attorney by the seller and it will not help you legally .Since your interest is to safe guard your deal and money paid, it is better to go ahead with an agreement of the indenting deal, which will take care of your legal rights till the execution of the sale deed and getting the possession.