There are a ton of great vapes out there; most the regular guys in here have more experience than myself. Poke around a few of the dedicated threads for the best info.
With my more limited experience, I have really only felt qualified to answer questions on the Plenty- a unit I have had about 2 years now, with the entire first year it being my sole device and my only means of medicating... we got to know each other intimately, to the tune of approx 1.5-2oz per month the first year.
I am unsure how warranties work second hand, that might have to be asked to each manufacturer. But most of the vendors you will find on this website stand behind their work. For the most part, these retailers are in the business of supporting their customers and will do what is needed to keep you running.
Next to the aluminum stem for the Lotus, the Plenty is one of the toughest vapes out there. Mine has suffered countless drops off my counter top, arm rests, coffee tables, etc. and all on hardwoods. There is absolutely no effect. I take it to festivals and power off my car, it has fallen in countless parking lots and even been accidentally stepped on by a 300+ lb friend of mine... this is a tank, meant to withstand constant use and abuse in coffee shops around the world- it can likely handle anything you can throw at it.
The minimal draw resistance was one of the deciding factors in me buying this vape originally.