When Diablo 3 is released, there will be expansions for it later on. You don't need to get the expansions if you don't want to.
Yes, Diablo is in Diablo 3.
It's been over 13 years since Diablo 2 was a true Hype, so this would give you a perfect reason not to worry if Diablo 3 would be your 1st Diablo game experience.
No, Diablo I has nothing to do with Diablo II.
No
You just need to buy the game. There is no online fee. It is free to play. Diablo 3 is free to play. There are no monthly fees unless you decide to use the RMAH.
No. There are Diablo 3 teaser videos on the Blizzard website.
No, it doesn't. None of the iPod-related Sims games need Wifi.
It really depends in some cases. If on the Xbox 360, no. But if you get it from Origin or steam you will not need wifi.
no
Mephisto is mentioned, but does not appear 'in person' during Diablo 3.
They are all good. You need to know how to use them.