Personally, I would stay away from Automagic unless you like cleaning white residue from all the emblems and between body panels. They basically make products for the detailers who clean cars for used car lots, so their products go on and offf quickly, so the shops can handle more products each day.
Like Short Cut said, for a shop that will deal mostly in privately owned vehicles, Meguiars and 3M are the best way to go. You can usually find a local distributer and when you buy in bulk, you save a lot of money. For example, a 16 oz bottle of Meguiars High Tech Yellow runs about $10 at an autoparts store, while through my Meguiars distributer, a gallon is about $28. For slightly less than 3 times the cost, I get 8 times the product. Both Meguiars and 3M also have strong support for their detail shop clients.
I use the same products on every car, regardless of the cleaning habits of the owner. I want every car I detail to look the best it can; besides, I'm a mobile detailer, so I only detail 2-4 cars a day anyway, and my costs per car is very low, even using Meguiars Pro Body Shop line of products (not that they are that expensive anyway). Also, since I only detail privately owned cars, I am not under the pressure of used car dealers or fleet owners to finish each job quickly.
Personally, I think using the same high quality products and the same techniques on <strong class='bbc'>every[/b] car will bring you more business and a better reputation. I haven't advertised in at least 4 years now, and other than when the weather (or current events like the 9/11 attacks that slowed me down for a few weeks) affect my business, I am always up over prior year, and 94% of my business is repeat or referral.
I have also found that a buffer is not necessary in most cases, in fact, I no longer use one at all. Buffers slow me down compared to waxing and polishing by hand. Others really like using them and can work faster with one, so at least try one, get proficient at using it and then decide if you prefer to detail by hand or machine.
Good luck. You will find detailing a rewarding job. Only one boss at a time (the customer) and you can see the results of your hard work in a few hours, not months like an office job. :up