Importing 3D Models at Runtime

I'm creating an application that allows users to import 3D models such as houses or buildings and explore them with an avatar.

My question is what is the best way to import 3D models and position them in the scene at runtime? I have already found the ObjImporter script on the wiki, but am not sure if this is what I need. I don't have much experience with 3D models and converting between the different types so any information on that would be appreciated. I would like to import the models from .3ds and .fbx formats if possible. I know that you can do this in the Editor, now I'd like to do it at runtime.

I realize that I will also need a GUI menu system.

Any help or sites that I can get more information would be greatly appreciated!