If you can get your business started with the cash on hand that you have, DO IT! Slow times are going to come (especially in this economy) and go no matter if you have $6000 or $20,000. However, if you borrow a sum of money, it's just one more bill (and a potentially large bill) that you have to pay, even during those slow times. The less bills, the better. Anytime you can do something without borrowing money, you'll be better off (big Dave Ramsey fan here if you couldn't tell).
Another thing I would suggest is to start simple. Don't attempt to "do it all" right from the start. Focus on basic services (ie - wash, clay, wax or sealant, interiors). These are the services that will allow you to "get your foot in the door" with a lot of people. They are also services that a) don't take as long to perform b) require less "work" for the money and c) keep prices reasonable for more customers. Offering basic services like this in the beginning will allow you to focus your efforts on quality and service from the get-go.
Remember that for most of us, it's the non-enthusiast type customers that are going to keep the bills paid. As you do these types of basic services, you can build up your reserves of money to purchase professional equipment, etc., allowing you to offer higher-level services (while still keeping the basic services available). When a customer with a nice car comes around, looking to spend some bucks on your best work, it's just an added bonus.
Since you have a full-size crew cab truck, you should have no problems fitting your tools and products into your vehicle. Everyone here knows what Scottwax can cram into his 4-door Maxima.
Another piece of advice I can give is to get a business plan written before you even start your business. Even if it's just something short and simple, you need to have a plan of action and goals that you're trying to reach. Otherwise, you are operating for the present, and not the future. As the old saying goes, "If you fail to plan, you plan to fail."
Lastly, good luck with your business. Have fun with it! That's why most of us here are in this business. We love what we do!