I created a script mod that enables custom mesh buildings in the game to replace the generated ones. Simply install it with the ModManager if you plan on downloading custom mesh buildings or additionally read the tutorial PDF that is included in the file if you want to create your own mesh buildings.
If you need any help regarding creating custom mesh buildings post here.
Download here (once approved): http://www.train-fever.net/filebase/index.php/Entry/154-Mesh-Buildings-Enabler/