My advice, Don't start up a detail business if you have a solid paying job now. My second advice is don't spend $20,000 on detailing supplies to start, that is just crazy. Look at Craigslist. There are SO SO SO many people who just like you, wanted to start up a detail business and go full time with it only to find out its a lot harder to get customers and make a living off of it. I can't tell you how many fly by night companies have popped up in my area in the last 5 years let alone since the start of spring.
If your gung ho about starting it up anyway, dealerships are not where your going to make your money at all. They are great for when your slow and need to collect a paycheck. They expect you to do it dirt cheap and want a lot of stuff compared to what they are willing to pay you for. I personally don't do dealerships because most of them have in house guys and pay min wage and any contract work, they pay no more than $100 for a full detail. And when I say full, I mean clay,polish,wax, dress, wheels,tires,engine, steam clean interior,dress/treat leather, stains, everything and they want it done quick and cheap and perfect.
If your doing this solo there is no way your going to keep up with the volume the dealers are going to suppsoedly supply you with.
If your still set on starting up, I would look into getting the basics and seeing how strong your market is first. $20,000 is a crazy amount to start with especailly if your looking for training and haven't been doing this full time. Its back breaking work and gets boring doing it day in day out. Yes there are a lot of times when you see your finished product and have a sense of pride, but cleaning up after dirty soccer moms and fixing paint htat someone washed their car with a brillo pad gets old after a while. And people are cheap for the most part, they don't wanna pay for what you fully do.
Another thing that you have to think about, I read all the time "I know I will do good because word of mouth will spread, I'll get a web page, and I know a lot of people". Thats great. If it were so easy to have a successful detailing business everyone would be doing it. Think of it this way. Lets say you work 5 days a week. You average 2 cars a day. Thats 10 NEW customers a week. which in turn is 30 new customers a month. Now those 30 new customers (which your really lucky if you find 30) won't need a detail for at least 3 months if not 6 to a year. So next month you have to find 30 more new people. And so the cycle repeats.
I've been doing this for 12 years. I still have a hard time finding new customers all the time and there are times where I am flat out dead. I do work for the local police department, major league baseball team, major companies, and numerous local customers, and a lot of body shops. I still have down time.
Sorry for the rant and I'm not trying to kill your dream at all. I just get tired of reading everyones posts of "I wanna quite my job and start a detailing business". Its like detailing is some magical business that will make you rich and is super easy to start and will just build itself. It won't.