I don’t think that there is a real guarantee but as the /e/ project just started and still is in beta phase, I’m pretty sure that it won’t just disappear in a few years.
Again, there is also no real guarantee for long term device support but as long as the device is supported by Lineage OS, it’s about 99% safe that it will also be supported by /e/. And who knows, if /e/ continues to grow over the years, they would be able to hire full time developers and further extend their list of supported devices.
All refurbished phones still come with a battery capacity greater than 80 %. Check this post: New Samsung S9+ from /e/ store is reading at 80% battery health according to Accubattery
Anyway, if you want to deep-dive into the OS-flashing world, I’d recommend you to not buy a refurbished phone directly from /e/ but from a reseller directly in your country as you will save quiet a bit money. But in return you’d have to take care of the software yourself.