Hi and welcome! Nicotine itself can be bad for your gums. I just try not to aim the vapor at my gums.
Don't buy random crap on the web. There are lots of "name" brands with bs "testimonials" and fake review sites asserting they are the best. A sure sign of a site to avoid is any ecig site that does not sell
eliquid and blank carts, or any that charges more than $20 for a battery. But there are also many that seem to be reasonable but sell crappy or incompatible models.
These are the most popular and recommended models, the the most supported threadings for adapters and blank cartomizers
Here, in order of battery charge life
Joye 510 (180mah)
KR808D-1 standard battery size (ala Vapor4Life in colors), Joye 510 mega xl (280mah)
Ego or Riva 510 (fatter batteries, 650mah+)
180mah is about 2-3 hours for a light to moderate vaper
280mah is about 60% more
650mah is is 6-8+ hours and there are larger batteries available for Ego and Riva
The KR808D-1 is a dedicated 2-piece (battery+cartomizer aka carto) but can be use with some atomizers (1 w/o an adapter needed) if you want to use it as a 3-piece. there are lots of prefilled cartos for this model
510 ships by default as a 3-piece (battery+atomizer+cartridge aka cart) but there are lots of blank cartomizers for it (just a small selection of prefilled ones). With an adapter, it can use KR808D-1 cartos
Prefilled anything is just a slight convenience. You will be adding
eliquid to carts or cartos. Carts can last near forever or until they get loose - you can replace the stuffing. The atomizer is the part that may burn out or get weak and you need to keep spares. Cartomizers are dispoable atomizer and cart in one piece. If you keep one damp you may get 4-7 days use out of one. Cartos hold more juice and need juice added less often .