Well re 2-part and 3-part you should try to figure out if cartos are the way you want to
vape. There is no "you should get a 2-piece" unless you are reading some site that only sells a 2-piece. I will tell you the cartomizer of a 2-piece absolutely does not last as long as a pack of cigarettes (in case you have seen any site saying that - typical rebrander lie).
With cartos it's a bit more complicated to fill and take a bit longer to fill, but a fill lasts longer. Cartomizers also eventually burn out or get singed - you may get 1-12 refills out a carto if you don't burn it dry, refill while it's still wet etc. But they are designed to be disposable so they do die so you have to keep a stock of cartos.
With a 3-piece (ie battery, atomizer, carts) if you take care of the atomizers they should last longer than a 5-pack of cartos (which about what an atty costs). Cart filling is easier but need to be done or at least topped off with a couple of drops much more often.
The most popular starter ecigs are
Joye 510 (3-piece but blank cartos available)
DSE901 (a 3-piece with a softer hit)
KR808D-1 (2-piece)
Some adapters exist between those models. I think they are all about 120mm long and 1mm thicker than a Marlboro.
