I've done some rough interiors as well, take your time. On more than one occasion the seats have come out of cars but I'm fairly comfortable doing that as a full time truck/car technician. Steam is a great tool, I have this one https://www.amazon.com/gp/aw/d/B00G00BTEA/ref=mp_s_a_1_2?ie=UTF8&qid=1479780256&sr=8-2&pi=SY200_QL40&keywords=mcculloch+steam+cleaner&dpPl=1&dpID=51-VKWDUxoL&ref=plSrch
I always tell people to remove trash and anything they don't want me to see. I remove everything from inside that's loose, do a first pass with vacuum. Extract/shampoo carpets and steam seats and whatever else needs done. Then clean and treat plastics, glass, seats back in and finally vacuum again.
It all really depends on how bad the interior is and the owners expectations/budget. You have to be realistic as well, I had a customer that I explained that an ink toner that leaked on to his tan leather seats and sat there for a week is probably not going to come out 100%.