So after several years of eyeing up the issue, I’ve finally bought some filler, high build primer, paint (flat white so no lacquer needed I believe?) and various grades of sanding pads.

Any tips/tricks/scare stories before I jump in?

Used to do this for a living many moons ago. It’s flattened the radius on the top two gouges so would require knocking out from the inside to enable you to maintain the radius. I would use a a metal drift but you should be able to fashion something from wood to do the trick. Slightly smaller radius than the inside of the van corner. Try to get it as good as you can as less filler is better. Sand all around the general area with ideally a rubber sanding block but a piece of wood will suffice. If you just use your fingers you won’t get it flat. The panel needs degreasing before you fill and paint it. You could roughen to surface area to help the filler adhere, with a sharp pointed scriber or similar. Put the filler on and sand away. The finish of your sanding and filling needs to be as good as possible before you paint is applied. I would paint up to the vertical seam on the side and down to the hinge area unless you can blend the paint into the surrounding area. Good luck 😉