Here's what I do.
First off the lexan paint colors used all depend on the final top coat color. If I'm going with a top coat that matches or is similar to a lexan paint I use similar colors, other wise I use grey. Silver is OK but it doens't give quite the metal look your thinking.
I spray the inside first, I prefer pactra, Tamiya Polycarbonate is a nice paint but the cure time seems much longer, but they have some nice primer greys.
I than use pactra to paint the outside, this color really doesn't matter, if it's scratched it will be scratched to lexan.
I use Rustoleum Auto Primer, but Duplicolor Primer works, just make sure it's a laquer.
I than top coat with Duplicolor.
I prefer Rustoleums new Clear Laquers to Duplicolors Clears, Duplicolor requires quite a bit of wetsanding and polish to make it shine.
and that's really it.
You can also just clear over pactra paint on the exterior, I only use duplicolor top coats because they have a variety of factory colors.
Any flats or satins need to go after the Clear. So think about your paint scheme, plan it all out and make sure to read the manufacturers recommended cure and dry times.
Oh and Rustoleum Hammered is one of the best non lexan paints I have ever seen, it sticks really well and is great for the roll bars and bumpers on HPI bodies.
You will get scratches, any body will, what your avoiding is paint failure like chips or peeling, but it will happen if you crush the body, but thats inside and out.
When you get scratches, use some testors silver enamel, dab a bit into the scratch with a fine brush, use a bit of flat black and rust color to get that
just scratched look.
You can also mix inside and outside paint.
The blue is on the inside and the black and hammered on the outside.
or just go beater and not worry about it.