That's funny, I know a guy that does exactly that...work from a van, mostly at trackdays too. He started out just having the van, but now he shares a big workspace with a partner who runs a riding-school.
I talked to him last week about how the business was going, and he said it was doing ok now. At one point he was so stressed out , that he nearly gave up though. And this is a guy who has been a bike-mechanic for years, runs his own raceteam and can pretty much fix anything with a motor in it.
If you're better than your competion (or there is none) it's possible to make a living. If you're smarter than your competion, you will do OK but it will take some time. If you're both, and are willing to put in crazy hours, you're business will be a succes.
I just started my own company 9 months ago. I now work 4 days as an employee to guarantee an income, and 2/3 for my own business. I haven't had a day off or a vacation since.
Work has been piling up, but I can't say that i'm making much more than I was as an employee. Most of what I make is invested in the business, and it will take a while before I can safely quit my job. I didn't have any money to invest, so I had to make money as I went along. I think it's easier if you have some money saved.
But it's something I've always wanted to do, and I feel i've accomplished and learned lot this year even though it wasn't making that much money.
Some tips I can give you from my own experience:
1. Make sure your monthly costs are as low as possible. Don't rent or buy expensive stuff, and keep your personal finances in check.
2. Be honest. If you can't deliver on time, tell your client and they will understand. If you accept the work, but are too late they will be mad. Mad customers are no fun, and bad for you reputation.
3. Ask people for help. Friends and family will be happy to help you and it won't cost a thing. Don't be too proud to do it all alone. I have my stepfather doing my financial stuff for free, a friend did all the design/ logo and website for free, and I got lots of free materials and tools from former colleagues and even two of my ex-bosses have helped me!