Having intersecting geometry like that is fine, and since this is a very low poly look your're going for you can even do it with more parts of that mesh to get an even lower poly count. As for the sharp edges, do you want the mesh to me smooth for like the round part of the wheels and sharp in other places? or just look…
Thank you for your feedback! Now I have few questions: 1. With the wagon, I fixed some edges I decreased triangles count from 2.1k to 1.6, but here is the question: how to know then I must keep parts of mesh connected and then I can just make to pieces of mesh and insert one in other (sorry for my poor explanation, I'm not…