Joye eGo without another letter after it, I believe, does not refer to any specific model, as the letter after it describes what model it is. With that said, I purchased an eGo-t (tank) starter kit about 6 weeks ago. It comes with 2 eGo batts and 2 atomizers and plastic
tanks that attach to the top of the atomizer and hold the liquid. All eGo's have 510 threads which allow for endless attachments, cartomizers, atomizers, tanks, etc. The eGo-c is the newest upgraded version of the eGo that has atomizers with a seperate head that you simply unscrew to replace. Which makes it much more user friendly.
I have at least 10 different "attachments" for my eGo. So, yes you can upgrade endlessly, at your convienence. I spent over $150 on a mod that takes 3 diff voltages, and I still end up using my eGo as my all day PV.
I would highly recommend it IMHO. Just make sure you are buying actual, genuine Joye equipment. Everybody has taken the name eGo and slapped it on their own knockoff products. So make sure "Joye" or "Joyetech" (I forget which, but either way that will mean genuine) is printed on the side at the bottom of the batteries. It will cost a bit more, but it is well worth the quality and safety difference.