Sweet ride Spud, this is going to be awesome!
To do the headlining hanging down you could just use paper kitchen towel, I have tried it before on a rusty wreck and it worked well.
Soak towel in white glue but don't add much, if any, water so it will dry stiff and roll it out to get rid of the paper pattern. Also add a bit of acrylic paint to the mix, which gives a good base for washes / drybrushing later.
Whilst still wet glue the edges to the bus ceiling in more neat white glue and once dry it should look pretty good.
Cheers
Leighbo