1. well, if you mean like paranormal investigators, yes. if you mean Buffy, no. i've studied myths and the paranormal since i was a kid, never went hunting down a demon to kill though.
2. it depends which type you mean. i'm not an official investigator, but it's a hobby. 'slayer' on the other hand i am not, nor would i want to be.
3. if you wish to work with demons or study them, you can start whenever, just look into defense techniques like shielding before you try summoning or tracking one down. study is the key really.
4. not to my knowledge no. there are spells to help motivate you to train, but you need to hit the gym for them to work. for astral beings i'd look into strengthening your spiritual side, not so much physical as it is energy work.
5. glad the site could be of help to you, just keep in mind what you see on TV isn't the same as reality. while demons are real, they won't physically manifest. granted you might see a full apparition, but they're not physical beings. yes, they can do physical damage, but demon possession is rare, and it's more common for demons to be invited instead of randomly attacking someone.