There are a lot of versions of IL-2 for sale, The latest version is "IL-2 1946" it encorporates all the previous patches and additions. (If you intend to fly online you'll probably want the latest version. also a program called Hyperlobby which will let you find online servers!)
To completely up-to-date you'll have to several download patches, 4.08, 4.09 and the last one 4.10. I haven't bought the Steam version but I think it is completely patched up to the latest version (steam customers help me out here!!!)
If you are really new to the sim you will want a joystick. It is really hard to fly using the keyboard. If you get really into flight sims as a hobby there are a number of other controllers that you find useful and help you control the aircaft in a more natural fashion, Throttle(s) and Rudder Pedals would flesh out your primary controls
Many people are finding a device called a TrackIR very useful - it is a head tracking device hat allows you to look around the 3D environment. Some people have stated that they would rather have the TrackIR than rudder pedals if they had to make the choice. But it all comes down to personal preference, how much you get into the hobby and money available.
There are a number of different levels of difficulty in the sim. My advice is start off easy and work your way up.
On of the big limitations of Il2 is the training supplied by the sim is non-existant. Use youtube and other online resources to help pick up the basics or use friendly places like this to ask questions.
The imporant thing is to have fun.
Cheers!