Many of my gameplay experiments have no real art. I often open paint.net and create a few coloured rectangles of appropriate sizes, and start added descriptive text to the image if neccisary.
You can do the same with 3d, just open a basic 3d editor, make a rectangle around the size of the object you want to represent and save it. The art doesn't need to be intuitive or descriptive for such early work in a potential project and can be easily replaced later once the general gameplay is impressive enough to attract an artist, or to justify hiring one.
You can can also look around royalty free sprite or 3d model websites for content that you can use as more descriptive and appealing placeholder; even if it doesn't directly suit your theme or art style.
Starting a big passion project is fine, so long as you understand that it will take a long time before it becomes something that will be impressive; and that you understand that as you improve and learn more about game design or programming you will likely want to redo a lot of your work(which can be discouraging for some people, as it will feel like the project is just treading water for a long time(years), before you have the skills needed to make any real progress). In forums, a lot of people don't seem experienced enough to recognise this off the bat(or their posts don't directly demonstrate this experiance), so planning such a large project can be detrimental in the long run if they just end up discouraged and put off due to a lack of tangible progress; thus the common replies advising a different mind set/path.
I don't have any advice about an engine for you, most engines you find should be able to do what you want(with a bit of work, very few will support what you want off the bat unless you choose to make a mod rather than a separate application). It'll be more about finding an engine that you can learn quickly and understand well, it's just a tool after all.