Random indeed, but I will answer as best I can. Mind you I am no expert.
Lowest altitude a plane can legally go: 282 feet BELOW sea level. There is a small airfield in Death Valley California. Planes can legally land there if they have permision.
Maximum altitude a human can breath at: A human can breath at any altitude, you can inhale and exhale as much as you want. Of course if there is not enough oxygen you will die. Altitude sickness due to lack of sufficient oxygen usually starts around 8,000 feet altitude. This varies from place to place, and also with the physical condition of a person. Some people are acclimated to higher altitudes, such as people whose ancestors have lived on mountains for generations. Even those people can only survive for very brief periods* above 20,000 feet altitude without an oxygen tank.
I will add then that when flying you are usually required to file a flight plan that says how high you will be flying and what route you will take. The air traffic controllers will then verify that you will not be interfering with any other plane's flight plan. Once you are in the air you must stick to your flight plan, unless there is an emergency, or you will be in big trouble.