Don't recommend the iStick 50 watt.
I also don't recommend internal battery devices in general. Lithium-ion batteries last ~500 charge cycles. You'll have to replace your whole device instead of just replacing your power cell. Internal is OK if you're not ready to get the batteries, an external charger, and you want more time to learn about battery safety. But, know that it's more economical for you in the long run.
I wouldn't normally consider a mod a beginner device unless you have someone who can help you or if you're comfortable learning from videos, blogs, and forums. It depends on how comfortable you are with the tech. The industry is moving toward things like the SubTank and S-Box being into gear, but I wouldn't have been comfortable with such devices 4 or 5 years ago. With that said, the SubTank and S-Box are fairly intuitive.
Mini versus Nano. The mini comes with an optional RBA (ReBuildable Atomizer) and it's bigger. If you think you might grow into building your coils (or using the pre-wound coils) then go with the Mini. You can still use the pre-made coil heads if you want to. The Nano is smaller. If you feel that the mini is to a bit too big for you, go with the Nano. Kanger tanks hang around for a while. So, you could always start with one and pick up the other later.