From a vision spinner on forward, forget smaller than or as small as an eGo unless you really know what you're doing and can handle an expensive battery killer mini mechanical that has some serious inherent risks in handling. Also, forget anything bigger than a Nautilus on your vision spinner, you're already at the max you can get from your spinner with the BVC Nautilus. I know; bummer for the wallet and for pocket space.
The good new is; a bigger mod will deliver better even on your current Nautilus mini alone. eGo batteries are small and don't really deliver what you think you're getting at max power. A decent mod with a decent rebuildable tank (I agree that Kayfun style is the way to go) will deliver so much more. Drippers need a little more power than the average mod in my experience, but personal experience can differ.
I definitely recommend you start with a regulated mod (box style or tube style) and learn ohm's law gradually with a rebuildable tank. Read the forum, explore things one by one and watch many many youtube videos (recommending PBusardo for moer expensive mods, Rip Trippers for builds and Indoors Smoker for the less expensive stuff). Mechanicals are definitely for people who know exactly what they are doing because the risk of venting a battery is real. Regulated devices have on-board protection and programmed limits to prevent accidents and even meters to help you understand what's going on..
Best advice I can think of: keep reading and keep asking questions. There is no such thing as a stupid question - just stupid people who don't ask questions when they don't understand.
[EDIT]: I've been typing for so long... I agree that the authentic Russian 91% is definitely on top of all Kayfun RBA's. Watch enough videos and talk to enough people and you will come to the same conclusion. Some RBA's might be a little easier to build on, but that has nothing to do with the vape experience!
