Homeowners of Reddit that replaced your water heater, can you educate us on what you learned about the process?

I've got a gas water heater that is at least 10-15 years old. I want to be prepared for the time coming when it fails. I've thought of my own questions but maybe you guys can add to it other considerations. I've got a gas water heater (in MN)

  1. How did it fail? did it just stop heating, i've heard people mention horror stories about dumping water all over the floor
  2. Did you replace it yourself (especially gas users) and what was that like for first-timers
  3. If you hired someone to replace it what state/COL and how much was labor
  4. How did you choose a water heater? Did you let plumber choose, did you go to big box store, or a plumbing store

Thanks in advance for help