Capital gain on Property taken possession in 2000 but sale deed done in 2011

I purchased a property in 1998 (DDA flat) which was taken possession in 2000, it was a DDA flat for which we paid some money in 1998 (initial amount) & some amount was paid between 1998 to 2008 as a monthly installment to DDA. The property was later registered in 2011. We don't have all the receipts for the monthly payment made between 1998 to 2008, the allotment letter from DDA from 1998 mentions disposal value in it. I have reached out to multiple CAs and have been getting different answers, I am trying to understand which one is correct. 1.) One CA suggested, we will need to get property value in 2001 (as per circle rate or other govt agencies) & then calculate capital gains by taking CII from 2001. He said it doesn't matter if you have all previous payment receipts or not. 2.) The other CA is suggesting we can only claim capital gain from the time it was registered i.e. 2011, if we need to claim from 1998 then we will need to get all payment documents made to DDA. The first CA also suggested that we can show roughly expenditure of 70-80K every 5 years on property maintenance for reducing capital gains, he mentioned we don't need receipts for it. Could someone guide me on the best way on approaching it, and how to reduce the capital gain so I can pay as minimal tax as possible?