I also agree with those who say leave it as original as possible.These are rare boxes to find complete and in good working order. And as soon as you customize its look, you will eliminate any potential future buyers who are purists.
Now that you've cleaned it up so much, does it really need to be painted, or are you looking to paint it "just because"?
I feel like if you do paint it, that you should definitely try to make it look as close to 100% original as possible. But, from the pics shown, I don't really see any glaring reason why it needs to be painted. For a 30 year old, rare portable stereo that you found for dirt cheap, that thing looks fantastic!
If you really want to paint a box for the fun of it, I would recommend finding yourself a more common unit that is not as saleable as that one. Read "more disposable." Worst case scenario would be that you somehow foul up the paint job and inadvertently foul up the box. Do that with a real junker first.
The GE that OSS mentioned as being modded in his avatar is an easy one to paint, I did something similar with mine.... (painted it to match my 85 Benz paint, which was also getting painted at the same time). In my case, it was almost an afterthough on a box I'd pretty much given up on anyway (low resale value and mediocre sound to begin with).
SD's red M70 paint is awesome, but M70s are very, very common. I couldn't say the same for your Sanyo...
And if you look at the paint jobs that Frank has shown us, his M.O. is to put boxes back to their original condition as much as humanly possible.
Well, that's my 2 cents worth... Congrats on a great find!