It is up to the user to select appropriate cells, there is a batteries & chargers sub forum for questions & battery dialog, and mooch has blogs explaining all aspects of the cells we use in simple terms.
Where you buy is equally important. There are several highly reccomended vendors for batteries, my first choice is always --->
Batteries and Chargers
No vendor knows what you intend to do with the cells, your vaping style or amp requirements, so it is all but impossible for them to make a recommendation without knowing the details of each individuals usage.
Again, it is up to the user to ask questions or educate themselves on proper battery selection & basic battery saftey.
Internal battery power devices aren't really any 'safer' than external battery power devices.
With all batteries, as they are used & age the capacity is reduced, meaning even unused they slowly go bad over time, when used at their CDR more rapidly, when abused
much more rapidly.
When an internal battery device no longer holds a charge for suitable amount of time or will not charge you throw away the device & buy a new one.
External cell devices, buy a couple $6 new cells & continue to vape.