first off, if you are planning on a 560 for CoD get a 2gb version- but this may change with the patch. CoD really can use the biggest gpu's if you turn everything up.
can't say what you think are decent graphics but you should be ok as long as you are prepared to drop a little from max.
Offline gameplay has a few things that are 'less than perfect' but you can get some good experiences (imho). Good mission design is key here and that's a skillset that is getting there slowly as everyone gets used to the quirks of the mission builder.
http://airwarfare.com/sow/index.php?...ries&Itemid=73
might give you a taste of what's available.
Steam-afraid so- but the offline mode is relatively pain free.
Personally i think you'll enjoy it, you seem to have reasonable expectations. If nothing else it'll keep you busy for a while learning not to blow up the engines
- engine management is a big improvement on IL2