On Tesla used car website, Autopilot means AP1, HW1 with 1 camera and not upgradable to FSD.
However, be aware that as of this year, there is another new kind of Autopilot that is standard and included in 2019 Model 3. This new "Autopilot" has HW3 as of 4/12/2019, 8 cameras and upgradable to FSD.
Original AP is HW1
New AP is HW3 as of 4/12/2019
But Tesla doesn't call them "original" nor "new" so we have to figure out which AP this is.
Yes, EAP is upgradable to FSD.
But if I were to buy a used one, I don't just buy one listed with EAP only. I would buy one listed as "Full Self-Driving Capability"
On Tesla used car website, that means FSD was paid and Tesla will do whatever it will take to get you FSD function such as swapping out the HW2.0, HW2.5... for HW3.
That way, you can skip an extra step of how to get Tesla to take your money for FSD after you prove that you are an owner an Enhanced AP.
But since you are interested in technology, I would skip buying a used one and just get a new Model 3 and you'll be all set because you'll get HW3 as of production date 4/12/2019.
Good luck!