are the interor fabrics leather or cloth? Cloth stuff's always trickier to use if you haven't got the gear to dry it properly - the autoglym Foam stuff with the light blue label appears to work fairly well, but we've also got some stuff from good old trago mills (Imagine a pound shop superstore):
Hagesan fabric cleaner. That's awesome for getting rid of tired marks and things.
what colour are the seats? If you can get your hands on a wet vac, autoglym do a range of trade Chemicals (stain remover, super interior clean etc) that you dilute into a warm water tank. Not seen an interior that hasn't come up with that little brew yet..
As Kernow said a wetvac and the right products would work a treat but to be honest if your paying out to hire one and get the products then you may as well put your feet up and let a "decent" valeter in your area do it. It might take you a few calls to find a good one but it shouldn't cost you too much.