Update - 12 Jan 2021
I made some changes that I thought improved the look; I made the roof a dark fabric material, like seen in some of the reference pictures, I changed the colour of the bumpers and started adding stickers and decals.
I gave every texture set a basic material and now I can start adding details like wear and tear, stickers etc. I'm especially happy with the chequered flag pattern on the curtains, I think it's a really nice touch.
Added a very basic base material to the outer body so I have something to work from.