25-strong dev team using C# 6 right now. Painless upgrade apart from a few pieces of legacy tooling. Some patterns haven't really been adopted much (e.g. exception filters). C# 7 looks like more of a leap but there's always an appetite to stay current in our team and I'm sure we'll take the plunge.
At my work I do a lot with 3.5 still. We need to upgrade our version of devexpress. More recent projects use MVC with .Net 4.0 or 4.5 (but our IT guy is nervous about installing such new runtimes on the servers that don't have 4.5 already..)
We use VS2012
Feel your pain. My work laptop has VS2012 on it.
Last year we were still targeting XP.
You can easily use c# 6 with .NET 3.5. I know because we do!
You just need to upgrade VS to 2015. Language version has nothing to do with framework version!