>> From a modeling viewpoint, I think MOI pretty much covers it, right?

I don't exaclty know your needs ... MoI covers a lot of stuffs, I will not say everything ... The key is to use the best tool for the job. Sds/poly modeler can be handy for more organic things (characters, trees) with lot of high density details. Finally even in the nurbs area, MoI is quite young, so maybe some other tools have to be added to face more complex shapes.

>> So after looking at apps like C4D and even Blender, it seems that their feature-heaviness will cause a lot of initial productivity loss just due to having to learn all those extra, unneeded parts (beyond just materials and rendering)

Well all you have to learn is those apps is how import you model in, how assign materials, how edit assigned materials ... placing lights, a camera and hit [render].
Depending of the render engine used, it can take some times to understand all the settings, and how to use them.

So basically you don't have to learn the complete package to start you render job.

>> With that in mind, can anyone recommend a quality, yet easy to use, renderer (with materials)

There are not many standalone renderer ... maybe you should have a look to hypershot : http://www.bunkspeed.com/hypershot/
Nice product for fast pack shot setup ... I'm not really convinced about architectual render job, especially interior renders.

>> or is my lack of experience, again, not going in the right direction with this and I should really get a full-featured app?

From my point of view, yes you should. Pre-made materials/renders are cool a couple of months ... but I'm sure that at one moment or
an other, you'll need more controls over the setting, more custom materials using specific textures that are not listed ... maybe you will need to add trees in your shot,
grass on the ground, etc ...