2 months ago, I bought a 70D with a 75D battery with 72,000 miles. The previous battery needed repair around the 65,000 mile mark. The original owner took it in and got a "loaner" 75D battery that Tesla ended up just letting him keep. I've only had mine for 2 months, but the original 8 year unlimited mileage power train warranty is still good, as it would be with your deal. My "new" battery charges at 93kW and I have zero reasons to worry. I don't think I'd worry about having a new battery given Tesla's warranty.
-Jim