A couple of quick thoughts on your questions-
I don't sand anything that doesn't need it- no point in making more work for oneself. That said, I might buff the entire surface of a model with a 1200 to 2000 grit paper if the surface is rough or pebbly, or only on certain panels or sections to give a slightly different texture than on surrounding areas. Since your working on a car, then you might need to buff the plastic to high luster so that the following paint has the smoothest surface to cling to. Smooth surface is essential to high-gloss finishes.
As for the paint... sounds like the paint is too thick to use under normal pressures. I've been thinning paints down more than usual and using more light layers to build up color. Tamiya paints can be tricky to fine-tune to one's tastes, I find that Model Master paints are easier to use and more consistent in their thinning.
If paint adhesion is an issue, then wash every sprue and part with warmish water and detergent to remove the mold release agents and let dry before doing any assembly. Then , give the entire model a good wipe-down with isopropyl alcohol before painting to remove fingerprints and such.
You might get better responses if you ask this in the cars forum as there's some car-specific paint tricks that I don't use.