So the chassis is fully stripped and sat needing some love. What's the best primer, rust treatment and paint to use? I was thinking of using raptor to paint the chassis with.

I'd try really hard to get it shotblasted, saves loads of work/time and you'll know for sure what you've got in front of you.

If it is fully stripped I would get is blasted, possibly chemically dipped, and then galvanised.
If you are just looking to paint the chassis as is I am currently looking into corroless paint which seems to be quite good and get good write ups. Although I personally have not used it yet ti is what I am leaning towards when I get around to painting mine.