Driving for a fleet owner is a great way to break into this business. Keep track of all truck expenses like fuel, maintenance etc. as if you owned the truck. This will give some real numbers to work with when deciding to buy your own truck.
Continue to do research during your down time. You use the term "D model" truck. The designation "D" is used in the expedite industry and can vary somewhat with different carriers. It is a measure of maximum freight capacity, i.e. length and weight. You should also go online and look up the differences between a Class 7 truck and a Class 8 truck. That alone should help you understand why nobody here has given you an average price for a truck.