The S60 you mentioned isn't horribly priced as long as it's in good condition (no excessive wear and tear, etc) and still has most of the battery warranty left. Basically it looks like the owner is asking a bit more than what they would get trading it into Tesla. Nothing wrong with that as long as you get it looked over prior to signing since there is no CPO inspection. Has the yearly inspection been performed? If so, did it get a clean bill of health? If the inspection hasn't been performed then I'd suggest having the owner get that done as a condition of the sale prior to signing. You could split the cost with them, etc.
As for the model (S60) itself, I have a 2016 S75 (originally a 60) with AP1. I don't see AP1 getting any major improvements in the future but it's pretty darn solid as it is. I wouldn't hesitate to purchase another AP1 if I was looking for another S.
All that said, your money will go farther with the model 3. You're in CA and have a launch day reservation. So you'd be towards the front of the line.
So really, I think this decision is down to preference. If you prefer the model S then the car you found seems to be a solid option. However, if something like range is highly important and you don't want to spend the extra $ to upgrade the S60, then I'd hold out until a larger battery model 3 is released.
