I would go with a dual battery mod. I absolutely love my Sigelei 75 TC because of the small size, but being a single battery mod, I go through batteries quickly. When I was using the Freemax Starre Pro on it at 45 watts, one battery would basically last me one tankful of juice, so I was going through 2 batteries a day. Since I started using the Velocity RDA on it at 65 watts, I'm going through 3 batteries a day. You may want to consider the Sigelei 150 TC. The IPV 3Li is pretty highly thought of, so that would be a good choice. The Sigelei 150TC and the IPV 3Li has basically the same chip. The advantage that the IPV's have over the Sigelei's is that they are firmware upgradeable, whereas the Sigelei's are not. Recent upgrades added a titanium setting, while the Sigelei is stuck on nickel only (although you can just account for the difference in the wire by manually accounting for it in your temperature setting).
I am not sure of the difference between the IPV4 and IPV4S. I thought I had read that the IPV4S came with the titanium setting, but if you have the IPV4, you can also just upgrade the firmware to get the same thing. I have no experience with the Cube II, but I do have one on order and it should be coming in tomorrow. When you mention the IPV2, did you mean the IPV D2? That is a pretty small mod too, so I think it would suffer from the same low battery life that plagues the Sigelei 75w.
FWIW, all I use now are TC mods. I have the Sigelei 75, Sigelei 150, Snow Wolf 200, evic-VT, rDNA40, and the SX Mini M Class. As I mentioned, I have the Cube II on order and I also preordered the Vaporshark DNA200 although I probably won't get that until the end of September.
As for the wire itself, you can go with anything from 24 to 28 gauge ni200 wire. It really depends on how wide your build deck is and how large you can wind your coils. Being so low resistance, you would need to do more wraps to get the resistance high enough....especially if you are doing dual coils. I've done dual coils with 26 gauge on my Velocity RDA, but each coil was pretty wide at 12 wraps and that came out to be 0.08 ohms. A single coil 24 gauge build at 12 wraps will come out around 0.08 ohms as well.