I have been in businews for fifteen years, and like to think that I do okay at it. Just some observations of mine.
1. Get a pressure washer. If you get a hot water washer it will open other options of revenue production to you. Some of them are FAR more profitable than being solely a detailer. If you get a fleet of cars to wash every week, you will have a good residual income, and a larger pressure washer will go a long way to making it more tolerable to you. this should cost you around $1200. If you go hot it will cost you around $5000, if you shop.
2. Get a Good generator. I have a little Honda 2000eu that I use for detailing, and a larger 5000 watt unit. 9 times out of 10, the little Honda gets the call because it is so quiet it makes it a pleasure to work. all the others are okay, but the quiet generator will make your customer happy. I can get them for under $900 local to me.
3. Get at least two different types of polishers. I have a PC 7424 and a Sioux Rotary. There are some thign sthat the PC just cannot accomplish efficiently, so the rotary will get the call. The PC should be around $110 and the rotary, depending on brand is around $150. My sioux was close to $250 15 years ago, but it still is going strong.
4. You want a small but powerful vacuum. I like the Ridgid protable. 5 hp and 2 gallons. It works excellent, but I had to put a higher quality hose on it. The Vacuum is around $70
5. Unless you are doing high volume, a bissell little green machine works great. If there is a stain that you cannot get out with it, get a scrub brush. It will come out. The bissell is around $80
6. Have one or two extension cords, for obvious reasons. Figure around $40 for both extension cords.
7. A good step ladder. Not a painters ladder, but a step ladder with largfe stand on type steps. The link is to the ladder that I use. It is expensive, but I have had mine for about 6 years now, and it is still in perfect condition. I would rather spend $180 once then $40 fifteen times.
http://www.littlegiantladder.com/lgl/control/product/~category_id=SAFETY_STEP/~product_id=STEP_BAR_4 They are $172 from Little Giant.
8. A DI or RO water treament system. Even if you are just washing one car, it will make your job so much easier and faster if you demineralize the water. It is also easier onteh wax that might be on the car, and there is, in my opinion, less chance of damaging the paint job. I rent mine for $70 a set.
9. Two copies of MSDS sheets. One for your truck or trailer and one for your office. If you have a trailer, then get three copies of the MSDS.
10. An open mind. Try to learn as much as you can about what you are doing and not just about how to detail. Look for oddball stuff that you can do with your equipment to increase revenues.
The total cost for all of this stuff is under $7000 with a hot water washer.
These of course are just my personal opinions. There is no magic bullet for everyone that will make everyone successful. Although it is good to have the best chemicals and MF towels and gee whiz products out there, they are not necessarily the things that will make you money. Watch your peenies and make every expenditure justify itself. If you cannot see a solid return on your investment, no matter what it is, it is not worht buying. There are a lot of things that I buy for my own truck that never sees the light of day on my customers cars, because teh return on the investment just does not make it feasible.