1. Yes the flashlight you got has built-in low voltage protection.

2. The best, no compromise, inexpensive 18650 cell is the Samsung 30Q:

It has high capacity, and is powerful enough to use all other higher power lights if you want, and will fit them all. Get 4 of them just in case, as you said.

3. For the charger, I would recommend the Folomov A4:

Can charge almost all battery types, from lithium-ion 18650s, to rechargeable AAs/AAAs, and can charge them relatively quickly if you want them to.

Just combine your order of Samsung 30Qs and Folomov A4 to save on shipping.

4. Good US sellers of cells/chargers:
Illumn: https://www.illumn.com/batteries-chargers-and-powerpax-carriers.html/?\_store=default
LiIonWholesale: Wholesale Li-ion Batteries | Liion Wholesale – Liion Wholesale Batteries
Mtn Electronics: Batteries