ipv4 is a great box mod.
So is the segelei 100 watt plus
Neither are expensive.
The sony VTC4 18650 batteries really are the best batteries for a box mod. Need to be bought in pairs, and married together from day one. Ideally should label them as a pair and always use them and charge them together. 30a batteries can provide a lot of power, just need to research how much. A decent two bay xtar charger shouldn't cost more than about 20, 25 quid.