1) Yes definitely. I stopped playing it because I have a lot of other games to play, but it's still a good game and I'm a long time D2 player so of course I want it :P
2) No PVP... yet. They're taking a long time building it, hopefully that means it'll be good. The internet is full of bitching about D3, most of which is total whiny QQ bullcrap, but the delay in making PVP is one of the few legitimate issues with the game IMO.
3) There is definitely single player just not offline. In D1 you played totally offline so people hacked their character to the point that multiplayer (LAN) was unplayable. In D2 Blizzard hosted the game online through the old BNet to stop that happening again, but because not everyone was online back then and LAN parties were still popular they made it so you could make a character either on BNet or on your local PC (which you could then play with others over LAN - ofc that meant everyone haxxored their chars so that sucked as much as it did in D1). But the important thing is if you made an offline char you could never play it online which totally blew, because obviously if you could do that you could edit your char files while offline. In D3 they just removed the offline/LAN option so you can only play online (otherwise imagine how pissed people would be when they spent all that time playing offline only to find out they couldn't play that character online).
TLDR: yes you can play single player, in fact most people do most of the time, but you have to be online. It's a lot more fun with friends though!
4) All depends what you want to do. Once you beat the game on normal theres 3 higher difficulties to do it on, and your gear keeps improving but it's the same content. Just like D1 and D2. There's achievements this time though which I liked a lot. Takes a while to beat Inferno (unless you just buy all your BiS gear on the AH I guess which a lot of people do these days... and then wonder why it's hard to find upgrades herpaderp). And if you like grinding for the best gear in the game or for gold/$ selling stuff on the AH there's Paragon Levels once you max out as well, doing all that could take a long time. Not even counting making alts - the different classes all play very differently.
In short, try it and if you like the gameplay then it's definitely worth buying.
Also given that it's PVE only did it hurt anyone that a few builds were OP for a few weeks? And at release the game was so brutally hard that "invincible" meant you could survive for more than a few seconds LOL.
---------- Post added 2013-01-11 at 01:06 AM ----------
Also Paragon Levels were intended to bring back some of that feel you had in D2 of slowly grinding to level 99. Which I loathed personally... but Paragon Levels are kind of cool :P