The home is sellable after it is remediated... The appraisal of the home and the final purchase price should factor the cost of making the home habitable again. There are quite a few variables when dealing with this issue: how extensive is the mold problem, the cause of the mold, the cost of removing the affected materials, and the cost of installing new drywall, insulation, roofing materials, and interior finishes. Remember, mold spores can travel throughout the home, even if the mold is confined to one specific area. You will need to have the home throroughly cleaned including the furnace and ductwork with a biocide to kill any remaining mold spores.
I would encourage you to speak with a Licensed Mold Remediation Company and a Licensed Environmental Inspector to get estimates on this home before making your offer and agreeing to a purchase price. You can use this estimate to your advantage when negotiating with the bank.
Best of luck,
David Jaffe-SRES, CDPE