Need advice - buying new flat in chennai

Under the concept of Due Diligence, directly, you need to be careful while signing any agreements. Read all the terms & conditions carefully and clarify whatevery doesn't satisfy you. Further, make sure you have proper receipt of any/all payments you make, specifying the context and purpose of such payment. Ensure you have the necessary agreements and they're registered at the right stages. 

You could further observe their terms and conditions in their brochures, since they're an established brand you say.

Further, while handing over the post-dated cheque, prepare a document, as I previously stated, stating that the purpose of the cheque and its absolute contingency over the successful transaction of the flat and whatever you deem necessary to put there. If they're executing the deal in honest intentions, they shouldn't mind leaving you with such form of assurance. (You could include terms related to a "good sale", i.e, where the ownership is transfered to you in absolute sense, with completion of all the formalities and execution as well as registration of all the documents.

 Agreeing with Mr. Arjun Kohli:

'Buyer Beware' applies to property deals.

It shall be appropriate to get proper legal opinion from a very able local senior counsel of unshakable repute and integrity specializing in revenue/property/civil matters before signing or making payment.

This may cost some FEE but can defend long term interest and hard earned monies.