Doc,
Make sure that you are up on all of the local regs for a mobile kitchen. If you buy new or buy used, you need to know if it will pass muster with the health dept and other agencies that may be involved. So save yourself lots of headaches and get to know the regs on being mobile better than the regulating authorities.
I have years of experience in a mobile setting, in motion picture catering and now with a kitchen trailer of my own. It was designed by me and built exactly to the specs that were approved by my state and local regulators. Still had problems getting it certified on final inspection. Seems that they have the ability to change their minds after approving the plans for the build. Cost me several thousand after the fact, and there was nothing I could do about it. I could not operate until the changes were made.
What type of food are you considering? Your menu will dictate the equipment and layout.
Do you want a truck or trailer? A brand new truck can run upwards of $200k, a trailer is much less expensive.
You can buy an old truck and have it re-worked mechanically and have a kitchen put in it.
The most common truck is a P-30 step van. You want to go with a wide body (8.5 ft) and as big as you can afford. I only went 20ft on my trailer, sometimes wish I had gone 24'.
There are plenty of used rigs out there, but finding the right one that has the equipment you want or need and that is mechanically sound can be hard.
Insurance is going to be very expensive on a truck.
You will most likley need to operate from a commissary.
Here is a link to Houston HD on mobile kitchens.
Good luck!