.7z isn't a model, it's a file compression method, you have to open the .7z file in 7-zip or winrar or something, then extract the files that are inside the .7z file, then open the model in your modelling program of choice, and export it to a collada .dae
If you can't figure out what 3ds format is, I dubt you can get another model to work ingame (Requires you to separate the mesh into different object, each having a different name, correct scaling and correct origin point).
http://wiki.beamng.com/Content_Creation Just export said model into .dae, UV map it, texture it and import into game.