INTERIOR - Stain removal depends on what the stain is. Grease/oil needs a petroleum product. Food needs a protein solvent. Coffee/soda needs something that can remove tannins and sugar.
Try a product called Folex. You can find it at Target, Walmart, and other retail stores. Its a good all around stain remover. You might have to invest in a couple of products to get everything done right. When I'm detailing interiors, I probably use 5-6 different products to clean it. Interiors can be a mofo.
EXTERIOR - I assume you will be working by hand, not machine (buffer). After a thorough wash, try a scratch and swirl remover, applied with a terrycloth applicator. Check out a product called Meguiar's Swirl X. They also make a product called Scratch X that is a little more aggressive, but plenty safe for hand application. It might take a few applications to make a noticeable difference, and don't be afraid to put some real elbow grease into the job.
When done, finish off with a good wax. I like Meguiars NXT on black cars. It will fill in light swirls and makes black look rich and glossy.
25 yrs as a car detailer, I've cleaned them all.