1. Depends on your wallet. It's probably ideal to get the PS2 controller/stick of your choice, a PS2 -> USB converter, and a modded PS2 with an "acquired" copy of the game. This way you can also "acquire" MBAC on PC and play it with the same stick/controller. Ideally, you wouldn't want to touch an old version to play competitively but MBAC on PC will let you play online and learn basics with netplayers before getting competitive. It also depends where you're located....if you're in NYC you probably shouldn't even bother with MBAC on PC at all since the scene is so strong there you can get comp. easily for MBAA. A modded PS2 can be done a number of ways but again it depends on your wallet and partly your skill with electronics; you could get a modchip for ~$30 and a used PS2 for really cheap and solder the modchip on yourself if you're confident, or you could do a number of other things like a hard-drive or usb-drive loader, a custom top with swap magic, etc. You're gonna have to find somewhere else for details on how to get that done.
2. As things currently are, no. Online doesn't exist for the PS2 version. There is some possibility in the future of MBAACC (for the arcade) getting dumped and someone developing netplay for it, but even that wouldn't be coming around any time soon. That doesn't mean you can't get quality practice, but whether you can or not really depends on where you're located and how active the scene is there.
3. Yes the scene is stronger in certain areas like NYC and socal. How long the game has left to live is a completely different story, but let's just say that if French Bread doesn't come out with some kind of USA port soon, things don't look terribly good. Nothing that is currently planned or likely to happen outside of an MBAACC leak with netplay (as mentioned above) looks to be giving this franchise a burst of new/more life, with the exception of a strong showing at EVO which is going to happen in.....two days. However like fiendmaw said the scene as it its strongest right now, the only thing is that there isn't much on the horizon to keep the scene going uphill for very long.
4. Other than MBAACC (basically an arcade port of the current ps2 version), no, not yet. Someone recently found in some article/blog that now that they've finished MBAACC they're apparently free to focus more time on [some other game]?
5. This game is kind of hard. Not Guilty Gear or Super Street Fighter II Turbo style hard where you kind of need to be a robot to land all your character/spacing specific combos and when you make a mistake you lose 50-70%, but hard because the game itself is complex and there are a lot of mechanics and like 80+ "characters" and there's really a lot to absorb just to be aware of what is going on while playing, and just being aware isn't even half the battle since you've got to use all of that to make decisions on what to do next and so on and so forth. And that isn't even touching upon mindgame aspects of fighting games like conditioning.
Come onto #mbaa on irc.mizuumi.net (if you know how to use IRC) to really get to know the community.