[TurboPascal7]

2012 is better than 2010 in pretty much any aspect. It's a lot faster, has better compilers and just feels/looks nicer. 2013 is a minor improvement over 2012 GUI-wise, but again a lot new stuff in C/C++ compilers.

The only obvious reason why you might "need" an older version of visual studio - there's a plugin with VC6 project file and you don't want to create a new project file yourself. I think 2005 was the last version that could convert VC6 project files to something newer. You never need an older compiler. Of course there is some stuff about "compiling plugins only with compiler used to build avisynth" etc., but as long as you don't do anything dumb (allowing exceptions to leave your plugin) - you'll be perfectly fine.

There are also some codebases that cannot be compiled with newer compilers because the authors abused some buggy VC6 behavior (leaking for loop scope being an obvious example). Do yourself a favor - fix them and not just downgrade to something inferior to current tools.

As for "how to create a new plugin", you just:
1) File -> New -> Project, there select Empty project in Templates -> Other languages -> Visual C++ path.
2) Go to Project -> Properties -> Configuration properties -> Configuration type. Set it to "Dynamic library (.dll").
3) Add avisynth.h to your project. Put it in the project folder in file explorer, and drag-n-drop from there to visual studio solution explorer or your project.
4) Add a new .cpp file, include Windows.h and avisynth.h in it.
5) Go learn some C++ and start writing code. Actually, please learn some C++ before you do the first 4 steps.