I would recommend you start out modding in Java, and then once you feel you have decent command of that, if you still want to try doing it in C# go for it. You'll likely have the tools to make it happen on your own at that point too, and could start assembling your own group around the project. Good luck whatever you choose!
Well, unfortunately, because of <thing you already know>, it isn't possible because <thing you already know.> As such what you want to do would require <doing things you don't want to do>.