As mentioned first by bonnie in this thread, the most important thing to do is isolate why you want one while remaining within the context of not driving much:
a) future collectible (untouched)
b) a roadster, that when I you do drive it, you want the best performance absolutely possible (modifications are fine, sport will be worth it)
c) just want a good reliable roadster to drive (midway between a and b)
For such a small production car, there are a variety of viewpoints on what makes the best roadster. It
is highly individual.
For example, if you want a max collectible, you may go for a founder's (or at least a 1.5) with
ultra low mileage, an iconic color and assume a possible reliability risk. For a more reliable roadster, you may want one that the previous owner drove a decent amount
and had problems sorted out with Tesla and upgrades/replacements installed.
I bought mine from someone I could trust with a great paper trail
who complained a lot when things did not work right and had items fixed early on under warranty. Not
a huge assortment of cars to personally see out in the northeast, so my options were limited, but
I am very pleased what i ended up with (I only had one restriction, it had to be a 2.0 vs 1.5 or 2.5 and a bright color
so I hopefully reduce my chance a bit of getting annihilated on the highway as we are so small). Just personal taste as
I am sure you will develop.
As with all cars, paper trail is key, know the repair and location history of the car. Be careful of strange gaps in history.
Then follow the other great advice in this thread and especially the forum. I read the forum 1-2 years before I bought mine,
it's required reading and the best education before you buy one
Enjoy the process!