Subdivision level on Subd Model?

Always collapse your model in the highest level that you would consider "render-ready". Unless you're preparing multiple levels of detail, you should save the model in the state that the user would just put it in the scene and render without bothering with smoothing it and checking if he/she needs more or less smooth. Most people are buying the models because they want to save time and that means they don't want to work on the model :)

Can always include both.

What I do is I upload a subdivided an smooth obj File but leave the blend File with a model that is subdivided n smoothed but I don't apply the modiFier

What I do is I upload a subdivided an smooth obj File but leave the blend File with a model that is subdivided n smoothed but I don't apply the modiFier

I always disable the smooth modifiers before OBJ export.
Smaller file and client has options, if collapsed then no options are left except when he hase smooth reveres software capabilities.

Just group the SubD meshes so they can be selected at ones to apply the modifier.

Regarding the condition of mixed polygonal parts, just tell about it in the description and render wire frames showing the condition of the topology.

You would always want to be staid with everyone and do not provide them with unpleasant surprises, that would be bad for your reputation.

Thank you for answers.
I appreciate it.

I will speak from the blender point of view as I am not very familiar with other programs and not sure how they behave in this way. In Blender you may have problems if your model has >1000000 vertices (very roughly) that viewport is VERY slow and not responding. So, what you should do in large scenes is, that you select your modifier to be rendered but not to be viewed in viewport, so you can switch between objects and move through scene freely but when you hit render, it will be with the modifier, so your costumer doesn't have to bother about it.