A Starter Kit and some E-liquid are all you’ll need to make the switch from smoking. Starter Kits usually come with 2 batteries so that they can be swapped over when one needs re-charging.
Spare Batteries are a good thing to have around just in case (remember you can’t just stop at a servo and grab one if one gets damaged or misplaced). Bigger batteries have more storage capacity (mAh) and therefore run longer before needing to be charged again.
Most standard batteries output 3.7V regardless of capacity. Variable Voltage batteries provide a greater degree of user control and usually can be set from 3.2V to 4.8V.