Client mods, plugins, extension

  1. There are client mods, plugins, extension?
  2. I`m C# developer can i create client mods, plugins, extension? There are any SDK?
  3. There way buy and set server on own windows hosting or only rent?

No there are no client mods, no there is no SDK yet, no you cannot download the server anywhere legally for self-hosting unless otherwise granted by Facepunch/Garry.