Pam,
Here is my take on your situation. When I quit smoking 2-1/2 months ago and started vaping exclusively, I started with 24 mg juice and this easily helped me curb all cravings for cigarettes (I was a pack a day smoker) and I was vaping not that much, about 3 ml a day. I found that the first couple of weeks, when I would vape heavily, I definitely noticed that I was beginning to overdo the nicotine. Being that you're coming from 3 packs a day, it sounds like vaping heavily at 24 mg would be just about right for you. Currently, I have dropped my nicotine content by mixing the 24 mg juice with a 6 mg juice at a 50/50 ratio, so that gets me around 15 mg. With the lower nicotine content, I find I am vaping a bit more, so now I am going through around 4-4.5 ml a day. All of the juice that I have ordered are 50/50 PG to VG ratio, except for the Halo Subzero menthol juice that I am using (Halo's ratio is proprietary so they never say what that ratio is).
I too started with the eGo style batteries and just a week and a half ago upgraded to the iStick 50W and the Subtank Mini. I also thought that I would never get into a box mod being put off by the size, but upon actually seeing the iStick in the store and holding it in my hand it wasn't as big as I thought it would be. And the fact that the battery capacity is HUGE helps as well. My eGo style batteries were 650 mAh batteries and they were good enough to last me one day. I had two batteries in my starter kit and I would simply swap them out every morning and throw the other one on the charger. With the 4400 mAh capacity of the iStick 50W, I can go days without recharging. I last charged it on Thursday afternoon and after 3 days of use, the battery gauge shows more than half remaining.
As for the Subtank, Subtank Plus, Subtank Mini and Subtank Nano, the only real difference is size. The Subtank is being replaced by the Subtank Plus and the others get smaller in size. The larger the size, the less often you need to refill it. I've kinda stopped using the Subtank Mini since it vapes a bit on the warm side and I prefer a cooler vape, so I am using some other tanks (Protank 3) running coils with slightly higher resistance. With the sub-ohm tanks, the understanding is that you can go with a lower nicotine content since it produces more vapor so it delivers more nicotine.
I still have, and still use the eGo style batteries. When I am out and about, and want something a bit smaller that is a little less conspicuous, they work great. When you place your order for a tank, whatever you end up deciding on, order some replacement coils as well. Depending on how much you vape, the type of juice you vape, etc. the coil can last you anywhere from a few days to a few weeks. If you don't find yourself comfortable in rebuilding your own coils, you will need a replacement coil.
If the iStick 50W is still a bit too big for you, you can always drop down in size to the iStick 30W. It is the same thickness and height, but about half the width, since it goes down in capacity from 4400 mAH to 2200 mAH.