Currently, it seems you have 3 major objects as part of the model. You have the steps (seats), the ground platform, and the pavilion or shell. They are all very low poly so far (you could subdivide them to help add more details, etc.). I would recommend keeping those 3 parts as distinct, separate objects. You can join them as one object, but they would still remain distinct sub-objects or elements of the model or mesh. Also the stones or bricks you've added to the seating could be joined all together as one model, but still separate meshes. I wouldn't recommend trying to "weld" all those elements together into one solid mesh unless you were planning to 3d print the object. There are many reasons for this. If you are concerned with excessive polygons which will never be seen, those are issues you can handle later on if need be. You can delete polygons which are intersecting other polygons or which are beneath the model out of camera view, etc. Since you mentioned this is your first real production model, I wouldn't worry too much about polygon count or hidden polygons. Those are more advanced things that either require pre-planning and a bit more experience to avoid.

For now, I think you're on the right track here, and as long as you aren't expecting too much out of your first attempt, I think you'll have a very easy time with this one. Unwrapping and texturing might be the hardest parts. For now, I would suggest finishing all the modeling first. Then you can join all the parts into one single mesh (no retopology required).

Joining the models will make them all one object, but they will remain separate elements in edit mode. To do this, you can select all the objects, then CTRL+J on the keyboard will join them into one model. Then when you go into edit mode, you can work on each part as a separate piece. This can come later on, depending on how many objects will be in this model. You may wish to unwrap some of the elements before you join them, but it isn't necessary.

I hope that wasn't too confusing. I may be rambling a bit. I could show you what I mean in a sample video if you need it. Just let me know.

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