I'd agree on purchasing a backup, esp. with that battery. If you do want an internal battery with longer run time either the zelos 50 or 75 would work, both fairly inexpensive. Technically, you can run a MTL tank on almost any battery but the more wattage you use and the more you vape, the more battery "life" you will need. If you do purchase a backup setup, lots are available. Nothing wrong with using an internal battery, just know it will have a shelf life, depending on how you treat it, how much you use it (most batteries can only be charged so many times, that does include external batteries, but you can keep your mod-- though some of those have shelf lives as well) and how the USB port holds up. Internal batteries can be easier to start with, as you aren't needing to buy batteries, an external charger, etc. That said, I use a two battery external battery mod with my MTL setup, I just enjoy great battery life.
One thing, most external battery mods are "regulated" mods, that means they will calculate the recommended "draw" from the battery, it's not a fixed draw like an unregulated mod. They're still called mods, well, IDK why. But unless you buy a MECH mod, you won't have safety issues in the same way as you will if you buy a REGULATED mod, so that safety issue shouldn't factor into your calculations, though if you have two decent internal battery mods (one just for backup, or alternate both) you should be fine to start. No need to overcomplicate things. Most people DO recommend at least one backup (mod AND tank) just in case something goes wrong and you can't vape one setup for a bit.
Best of luck and welcome to ECF!
Anna