Are async/await keywords only usable with VS 2012+ and not usable with the C# compiler, which I could use from command line?
No, it's fine to build with csc.exe from the command line, so long as it's the C# 5 compiler (as shipped with .NET 4.5).
If you want to develop in Visual Studio though, you'll need to get VS2012 or later. (I really wouldn't use the CTP at this point - there were various bugs fixed in the real release, and there's no need to go through the pain of them now.)
Don't forget there's always the Express range of Visual Studio editions, which are free.