1S or 2S li-ion battery charging design for ESP32/Nextion device
I've a similar project ongoing.

My answers based on my research is:

Don't go with the UltraFire batteries there are so many fakes and the test I've seen of them is that the Ah is far much lower than the specs! As can be seen here:
Batteries test various Li-ion types

Regarding one or two cells it's a lot easier to go with one cell. You will only need hardware protection (for safety reasons) for one cell instead of two.
Li ion cells are highly dangerous, so I wouldn't make use of Li ion without it. Not even for hobby projects.

Are you afraid of that your battery won't be able to handle the up time, change to 21700 batteries instead of 18650 much more Ah!