I would buy a couple of extra springs and a feed tube assembly. It doesn't often happen that you will need them, but if you do, it is nice to be able to take care of a simple repair quickly and easily. Add 3 or 4 extra 6ml bottles. If you want some atomizers to try juice with, the 2.0 ohm ones are fairly good, and reasonably priced. Also, the 1.7 ohm resurrector cartos are good on the REO.
The batteries you want will be the AW IMR 18650 AW IMR 18650 or the AW 18650 (2200 mAh) AW Protected 18650 2200mAh Don't go with the larger mAh batteries, they will overcompress the spring in the battery compartment. RTD vapor gives 5% off with ECF5