Deductions & credits

My gain will be about $600,000.  If an appraisal may not work and the mortgage/loan may not work, what exactly would work if I do not have all the receipts?  Not knowing, many subs were paid cash as that is what they preferred.  So, in order come up with a cost basis for my new home (at the time) what would be acceptable to the IRS?  That's all I got...

 

Thanks much in advance.