Cost-bounded sub-navmesh

Hi!

I'm currently working on a tactics-style game and I would like not to use a grid, but, rather, navmesh and actual distance/cost as a metric for how far a unit can move. I'd also want to display that range through some terrain overlay.

Re-stating the problem, I want to cut a sub-navmesh from the general navmesh whose mesh is actually the parts of the navmesh within a given cost from the agent.

Looking at the NavMesh API, it seems pretty bare-bones and doesn't really let me anything like that at all. I'm thinking of still leveraging navmesh generation but then just grabbing the mesh through CalculateTriangulation.

Performance isn't a major concern since I'll be doing one of those queries every few seconds or so.

I guess my question is two-fold

1) Is anyone aware of a full navmesh solution that would get me closer to that?
2) Failing that, can anyone recommend some maintained C# library for something like a half-edge structure/anything more pleasant to work with that the vertices/indices/triangles meshes Unity gives you back through CalculateTriangulation?
3) General tips/advice/algorithms to how to go about this? I have a feeling I'm not the first one to try to solve that problem.