Im no expert by any means but here are some basic answers....

1. It changes faster than the weather! But in terms of potential - massive worth!
2. Is anything a sure thing? For me Steem is closer to a "sure thing" as I am directly experiencing its benefits and using it on a daily level. But never invest more than you can afford to lose and be ready for the game to change without notice.
3. No, it is digital information. But it has real world applications which are very physical. So while the coin itself isnt a coin, it can be used as payment theoretically the same way a Venmo payment or PayPal payment could.
4. Not at all. You are actually building a crypto portfolio just by using Steemit. If you want to buy some Steem to power up your account you would just need to buy some Bitcoin first (easy through Coinbase) and then buy Steem with the BTC.