FiveOh,
What's up brotha? Here's the way I see it:
You're just getting started so having this opportunity could work out very well for you as you develope your business. It's not like you can just say 'poof', I'm in business. How awesome will it be for you to use the initial time with this job to get organized and running. It will also give you ample opportunity to hone your skills. And, it will also provide you with a shop in which to opperate in. Sweet deal!
DON'T GET SCREWED OR TAKEN ADVANTAGE OF!
You need to have a firm business relationship with these guys. You said they are friends and great, but you need to make them understand that you have a goal of having your own business. Since I know this is your ultimate goal, don't get caught up in thier cars all the time.
This is what I think you need to establish before starting:
1. You are not a clock puncher. Come and go as you please. If you are getting paid by the hour, fine. Let them know that you will be billing them hours for work done on their cars. This way, you can manage your own time and work on both your business's details and dealership's details when prudent. This, I feel, is vital to establish initially so that a professional relationship is made rather than that of an employer employee.
2. Let them know that you will be buying and maintaining all of the supplies for your side of the deal. This way they won't someday say that you are cheating them. Also establishes a proper segregation of the two deals.
3. DON'T OFFER THEM CRAP for letting you use their space! Why should you? They are already getting a guy who's detailing their cars for $10 an hour. And a guy who is making a career out of detailing. You may not, but I'm sure these guys do, know that detailers are hard to keep and a good one is worth a pretty penny to hold on to.
Let us know how it goes.