I found this to be a good lets play to watch. whats the exact Arma game I need to buy? And is adding the mod difficult at all?
You need Arma Comined Operations. 25 Euro on Steam.
And no it is super simple to install. I don't see why so many people have problems with it.
Here is a step by step guide:
1. Once you bought Combined Operations run them both at least 1 time before installing the mod. (that will create some files you need)
2. Then download the beta patch here - http://www.arma2.com/beta-patch.php (Latest build at this time=94364)
3. Once you have installed the beta go to - "C:\steam\steamapps\common\arma 2 operation arrowhead". And rename "arma2oa.exe" to "arma2oa-OLD.exe".
4. Go to "arma 2 operation arrowhead\Expansion\beta" and copy "arma2oa.exe" to the folder where you just renamed the .exe.
5. Now download the DayZ files here - http://uk1.dayz.nu/latest/ you can store them on your desktop for now.
6. In "C:\steam\steamapps\common\arma 2 operation arrowhead" create a new folder named - "@DayZ". In that folder great a new folder named - "Addons".
7. Move the downloaded files you stored on your desktop to the Addons folder.
You are almost there bear with me now
8. Now right click in steam on ARMA 2: Operation Arrowhead and select property's. And then select "Set Launch Options" In the launch option screen add this parameter - "-beta=Expansion\beta;Expansion\beta\Expansion -nosplash -mod=@Dayz".
You are now good to go.
Launch the game as Combined Ops and enjoy DayZ .