This is not a specific endorsement as I have not personally used their services, but with that being said, one of the TMC vendors (
@FindMyElectric ) has a very good write up on free unlimited supercharging, transferability, etc. You can find it here:
Does Free Supercharging transfer when you sell a used Tesla? What if you buy from a dealer? Or another 3rd party? Answer those questions and more here!
www.findmyelectric.com
I saw it referenced in another thread here. If you are looking for a car that has guaranteed free unlimited supercharging, you are likely looking at buying a tesla model S thats no newer than the beginning of 2017, and also one that someone can prove has never been through teslas hands at any point.
Any car with FUSC was also likely supercharged a ton, for whatever that is worth. Me personally, free unlimited supercharging would have been a perk on a new car I bought, but wouldnt be one on a used one unless I was driving it for business use and planned on putting 30k+ miles a year on it. Perhaps thats just me though.
TMC members are pretty knowledgeable as well, so you can ask questions here about things you dont understand or want clarification on.
good luck.