My wife has a treadmill desk. She walks. The noise in the room below gets annoying. But she is my wife.
You should check your lease and the local laws.
I think you should talk to the people below you before you purchase the treadmill. After you install it, you should go to the apartment below and see how it sounds while someone runs on it.
You might want to pay your neighbor’s rent.
Go outside and run or get a gym membership.