This is the project I wish to undertake. These are the products I'm in need of in order to produce this work. I was under the impression that this was a place where a Poser user could ask questions and get answers. Perhaps I was mistaken. I'm looking to create a complete city street scene with all of the elements which a typical street scene would contain and with all of those elements containing all of the most important elements which they do in real life.

I understand your quest. However, you must understand that what you are asking for is not a normal thing for 3D modelers to produce. At least inasmuch as the details that you are requesting be present within one model or crafted set of models suitable for the sort of rendering you wish to do.

However, you can find these sorts of things at various 3D sites, many of them for free. But, they will be of varying quality, of course.

For many of the small items you want, like fully modeled light-bulbs and such, you can find those in several places where people are uploading projects they've done for funsies or to practice their skills. For some esoteric models, like a fully modeled vehicle, you are not going to find them anywhere for free. Why? There's no practical use for such heavy geometry to be present in one model, just for general rendering. However, you could find such a model, but it would likely cost you a hefty amount of money because the labor involved is... a lot.

You can find free internal organ models, but many will be derived from CT scans and may not be at the resolution/fidelity that you want. You'll have to search hard for them and you will have file format problems with many, since they're not designed for non-professional use. However, if you can find a format that you can use and, more importantly, an object that you can actually texture in some way, you can probably adapt it to your use fairly well.

It would probably be a good idea to check Blender and Sketchup forums, where enthusiasts pump out a wide range of models. You will not find many mundane inorganic models in Poser or DAZ forums. Those programs are not focused on rendering inorganics. (Though, they can, but with varying ability. (Organic models, which these programs specialize in, are objects that generally represent natural things, like humans, cloth, plants and other objects that have "organic" soft-looking surfaces, with soft curves, etc. Inorganic objects are hard-surface objects, like tables, cars, airplanes, test-tubes, lamps, buildings, etc.. )You may find some direction for some of the more esoteric models, like the fully modeled engine/car, by querying various professional-level CAD and design forums. The chances of finding free models would be low, however, as anything like that would be purpose-built.

Some of the best scene construction can be seen in Stonemason's work. He's one of the only artists that specializes almost entirely on constructing outdoor/indoor scenes and his work is beautiful. You might be able to find some of his products that you could retexture to provide you a basic starting point. There are also a few outdoor scene products available by other artists, but I do not know the resolution of detail that you expect. There are several outdoor scene objects and resources available for free on ShareCG, but they may be of varying quality and resolution, so you may have to do some retexturing and add some objects to them for them to be suitable.

Lastly, there's always the option of grabbing a 3D program and starting work on the project, yourself. Looking for "exactly" what one wants is usually what forms the foundation behind the motivation for many learning 3D modeling.

PS - Don't get discouraged or upset. The sorts of things you're asking for are the exact opposite of what most 3D artists attempt to achieve with their work. The basic idea is to focus completely on the final, rendered, product and to construct the model in such a way as to fully accomplish that focused goal in the most efficient manner possible, not the most "realistic" manner possible. The concept of "Realsim" is applied to the final rendered product, not the 3D construction of it. Because of... reasons. :)