If you want variable voltage, here are the two I would suggest:
Less costly but also with less features: a variable voltage box mod model from Ken's Box Mods. Ken is well supported on ECF and stands behind his products:
Kens Boxmods .
One of the top of the line models is the Darwin from nhaler.com. It comes with a full 2 year warranty, will give you 21+ hours on a charge, has regulated, boost circuit technology which makes it the most consistent model on the market and doubles as a pass-through so you can charge and vape at the same time. It is the only power regulated, variable wattage model made:
Nhaler
There are a number of other good options, as well. You can use any atty/carto or tank on these models although LR type attys are not needed, obviously.