It is best to have 2 batteries so that you can use one while charging the other and don't have to wait in order to vape.
Good chargers can be around $20-$40 dollars but are well worth the cost. If you are using the 26650 battery in that mega pico, in a charger it will still take around 6-8 hrs to charge and more if you try to charge it while still in the pico via the usb port. Do you really want to wait that long before you can vape again?
As to the coils, the above poster has explained it well. You will know when its time to change the stock coil in your tank. It just won't give out the same amount of vapor and won't taste as good. Some just automatically change their after a week or two, depending on the flavor used. Dark ejuices tend to gunk up the coils faster, such as tobacco, coffee, dessert or really sweet flavors will do that. Clear ejuice won't gunk up as quickly such as fruits, gums, candy, drink flavors.