A carpet extractor is a must have if you're going to detail many cars. A good friend has a Mytee HP60 which is what I'd buy if I had an extra $1200 lying around.
Products like Folex spot remover and Resolve carpet cleaner have worked very well for me. You can get them at Lowes and Target.
Steam cleaners would be a great addition for a pro detailer. Good ones I've found cost around $2000. A decent steamer for an amateur will run you about $200. Anything less is probably junk (I know, I've tried a few).
Interiors require many speciailized cleaners. What works on greasy spots won't budge food stains, for example. I've been researching interior cleaning products for years and still don't have the perfect product mix. All I can offer is do lots of research and ask product mfrs and distributors many questions.