Any mod can be used with salt nic (or freebase nic). Some advertise that they're for salt nic, but it's really just hype.
Here's a tip for your search: SMOK is a major company with high quality marketing - and, seemingly, very little quality control on its products. Others are more reliable.
When you say you want to upgrade, what in particular are you looking for? Your answers will help us narrow things down. Also, are you familiar with the categories of mods and tanks (pods / MTL / DL), or would a brief overview be helpful?