First of all I'd be careful using plexi underneath the car in case it cracks. Maybe use some moderately deformable plastic like acrylic. And you can't lose with coroplast. I drove for months with the coroplast belly pan and haven't seen any damage. And it's cheap!
For the wheel thing, I was saying that lowering the car would more than make up for the increased area of the wider tires because lowering it decreases frontal area. The car's body curves in so much that I don't think any wheel fitment is really going to benefit you (unless you ran 20* positive camber with 18" flat face wheels). You'll always have the tires poking out the bottom and tucking in up top. For a car with flat sides like a kei car it might be worth the money/effort. For a pre-90's American car I really wouldn't even bother. Put your effort toward belly pans.
The spoiler was an idea. I don't know whether you could pull it off or even find one for that matter. It probably will look weird, that coming from a guy who likes the look of Wartburgs.
The "plausible stock" thing is what VW guys call "OEM+"- it looks stock, but with a few tasteful improvements. I'm not sure if other people use that term but it might help explain it at least with VW people
