Hi!
The poly count on our doubledecker buses will be about 40.000 including wheels and interior. Whether OMSI runs fine with a particular model is not a question of the poly count but of the number of materials (textures and sub-meshes) used. Of course less complex models will perform better, but usually the number of materials has a larger influence on framerates than the poly count. So you can of course build a 100.000 poly model as long as you keep an eye on the materials.
Depending on the purpose, we usually use 256², 512² and 1024² textures, the latter ones mostly for buses and large buildings. 2048*2048 and even larger will work as well as long your graphics card supports them.