OK, here goes :
1 - Not sure of this one, think the game plays till somewhere in 46. I also seem to remember some victory conditions concerning capitals. Best to check the manual or wait for someone who has been playing the game lately .
2 - Lookie here
3 - SC covers the European theatre only, so no world domination or other continents. SC2 will only cover the ETO as well.
4 - SC will allow you to play the Allies or the Axis, not individual countries. Diplomacy is not a big deal in SC, only declarations of war are possible and these will affect the entry of other countries.SC2 will have more diplomatic options.
5 - HOI I and II is a completely different game. SC is turn based, not as complex although not simple. SC really shines in PBEM or TCP/IP play against other players, there are really some awesome players out there.
The PBEM community is quite large, you can find them here.
The AI is quite easy to beat, even on the hardest setting once you get to know the game.
SC is a great game, I had many fun hours with it and the community here is awesome as well although things have slowed down a bit since everyone is awaiting SC2!
Welcome to the forum!