Get a new bike. You won’t be able to tell what’s wrong with a used one and you might end up shelling out more than you need to if you find anything wrong with it.
Read up on bikes and listen to some podcasts. Bikeradar and Global Cycling Network have really good content that you can use to educate yourself on everything from frame materials to the drivetrain and tires and brakes.
Mountain bikes are heavier because they have shock absorption and fatter tires. Hybrid bikes have decent tires and can work great for commuting. Road bikes are generally for going fast. Do your research.
Don’t turn up your nose on the smaller brands. They usually get shit on by people on the internet because most people who love to talk about bikes are enthusiasts and have different needs. They might well have the perfect bike for your purposes.