Folks, there are good stories and bad stories of buying cars. Please remember, we are not dealing with robo-advisor yet. People's skill set and follow-up motivations are different.
Back to the OP's questions. There's been some excellent replies here already. Here's my two cents.
If you are shopping for a '16 or later, buy only a '16 build only after Oct '16. This one has the AP 2.0 hardware. Those built after Oct '16 AND with FSD purchased then or now, will get the upgrade to the FSD chip. Even if you do not care for it yourself, - today, this month/year has an edge in resale value over earlier '16's.
If you buy from a non-Tesla dealer you will not be eligible to purchase the extended warranty from Tesla. If you are buying from private owner, ask that person to take a picture for you of his Tesla account where it shows their car is allowed/eligible to purchase the extended warranty. You want the choice to purchase it versus buying one that you are not allowed. Otherwise, you should be able to negotiate the price for the absence of the option to buy the extended warranty.
I don't see how any non-Tesla after-market extended warranties can operate or be practical. Personally, I don't think they would have access to Tesla repair cost records to offer a reasonable warranty. And I've seen no treads anywhere that report good or bad on Tesla after-market warranties.
If I found a car I like being sold by Tesla, I would work my deal to travel to whatever location it is and eyeball it, purchase it and drive it home. Besides matching your expectations to your goodie list, you have the adventure of driving it home. (Something we can't do where I live.) Makes a great beer-drinking story.