How do I make multiple Blender objects into one? Situation

I have obtained some blender files &amp; licences for 3d models of transit vans and some trucks. Currently, when I export these objects to .obj files they are rather large, take a while to render and look to have been built in segments.

Problem

These models need to be used on Android devices and they need to be observable through one .obj file. What is happening is when I open the .obj I am not seeing the same model as I do in Blender. The doors and wheels of the vans for instance don't appear.

End Product

I need to find a way of reducing these models to be just one single object. Items on the interior of the van don't matter, seats for example. The file would ideally also be around 200kb, and exportable in a single .obj file. Can anybody shed some light on how I would go about doing this?
Remove the objects you no longer want in your model, like the interior objects. Then select all of your objects that you want in a single model and hit Ctrl + J then click Join selected meshes. The multiple objects will now be one model you can export.

For further reducing the complexity of the model for better performance, you'll need to either remove more objects before joining them or you need to remove details from the objects you want to keep.