Simple In-game model designer. User generated content

I hope to eventually make a multiplayer game which allows players to design their own ships by a user friendly system of placing vertices, connecting them, and filling the panels with desired textures.

Another feature would be of creating custom weapons for said ships, but that's another matter entirely.

I've tried doing this project with Game Maker, but the multiplayer and punchthrough issues were just too much for me to deal with. Netcode is an absolute nightmare for me. I get the concepts of client-server connections and client side predictions and soforth, but building the infrastructure for it, and dealing with firewalls and routers just makes my blood pressure spike

My question is mainly... Where do I start with a project like this? To allow a user to create their own designs, and for that content to be visible to other players? I really just need advice on a first step as I haven't set foot in Unity's editor for several years.