A truck is nice because of the bed. You can easily haul large items, and you have plenty of room for tools and supplies. The downside to a truck is that unless you have a toolbox or a shell installed all of you tools are left unsecured and will be exposed to the weather. If you plan on doing any type of hauling for your business, a truck is a good choice.
Vans have many advantages as well. The main benefit is that you can lock up all of your stuff easily with a van. Also, if you live in an area with a lot of foul weather you will benefit from your tools being sheltered at all times.
Don't let the fact that you don't have a truck or van stop you from starting up right away. With a little planning and ingenuity you can make any vehicle work for you. There is a handyman company in Portland that does everything with a bicycle and he is doing it successfully! If he can run his business on a bike, you can surely run your with any vehicle you have available.