As far as degradation, absent any outliers, all Tesla's have shown probably less than 25% degradation. Degradation should not be a concern. All that matters to nothing if the HV pack fails due to non degradation (like mine did on my 2012 P85). Non degradation failure requiring pack replacement/repair would be like one bad cell/module, lose wire, faulty circuit board, etc.
You're going to have to accept risks when buying an out of warranty Tesla, or any like car for that matter. There is no solid data on failure of the different sized packs, just anecdotal information.
The hard truth is out of warranty replacement packs can be more than the car's worth. Once costing $100k+ USD, these are expensive cars to maintain. Set aside $10k-$20k for maintenance in an interest bearing account. If you can't do that you shouldn't be buying a car like this.