The model is looking good there!
As for the seats, I'm not experienced in this sort of things,
but you can either use the clay, or if you are up to the challenge
enough, you can maybe buy some pieces of velvet (to intimate the
material used for the seat covers), dye them carefully, then stuff
a bit of cotton or some stuffing and glue it to the seats. Not sure
if it will work though; I will let your experience call.
Just for kicks, maybe you can do a model of a Dennis Trident
afterwards if you feel like doing another model after you finish
this one.
ah ha..i'm thinking of using vacuum forming for seats and maybe
other things..will have to first make a clay/wooden mould for that
to happen..
i might be building another bus soon..still thinking of what to
build..hehe..Dennis Trident would be good tho.. 
