For getting the paint off, you have to know what the paint is based on (there are many different kinds of paint which need different product to get the paint off)
Thinner or something should work. Whitespirit won't help, it's not aggresive enough for dryed paint.
Best thing is to go to a paint shop and ask their advise. Maybe u can sandstream your case (easiest way) but inform just to make sure the stream of sand won't couse any problems.
I know getting paint off can be a hell of a job and some of those paint removers just make the whole thing worse. But if your planning to paint it again u can can sand the box until its smooth and even. Sand it up to 800 using u a sanding-path or another flat product.
If u decided which way to go but ur not quite sure, just ask