Hello Andrew,
Trevor has added the thought of an A frame mounted mover. These are excellent provided you have solid flat ground, but if you intend to use it on site, then you must be aware of the major shortcoming that A frame drives have, that is lack of traction on slopes and slippery surfaces. As a guide, if you cannot stop a van on a slope by hand (i.e without the brake), then under the same conditions an A frame mover is likely to loose grip, which will not only prevent it from starting a van on a slope but potentially worse it will not be able to control and stop a moving van! You also have to lift and remove the device each time you couple to the car, so consider how to store the device when towing.
Chassis mounted movers can also suffer loss of traction but usually under much more severe conditions. They are always available at home or on site, and most allow you to move around the van so you can see clearly any of teh obsticals you need to avoid.
The choice is yours - happy motor moving