You're asking questions that could fill a few pages. Might start by giving us an actual budget. What you look for in a $500 bike is totally different than a $1500 bike.

Personally, I wouldn't look at any hybrid without disc brakes, a 1X drivetrain (unless you live in an area with super-steep hills or you're in very poor conditioning), and something not too terribly heavy, like big box stores carry. There is some variation on the quality of wheels and tires on low-end hybrids, so this is something to look at. You should budget for replacing pedals and saddle (which are total crap on low-end bikes), and most people like to add bar-ends and a rack of some sort. You can't go wrong with any hybrid from Trek, Giant, or Specialized. Even entry-level components from Shimano and SRAM are generally good enough for most. My $0.02.

I'd buy new unless you are a mechanic (or know one) who can look at used bikes and make good evaluations.. And used bike prices are sky-high at the moment and often don't represent very good value.

- Mark