Since stage rallies usually take place in remote areas, at the minimum you will need a mobile radio for the 2 meter (144-148 MHz) band, as well as an antenna for your car. Just about all rally communications take place on 2 meters, so a single band radio is all you need. You can find these available used at hamfests or online for less than $100, though a new radio like a Kenwood TM-281A or Yaesu FTM-3100R cost not much more than that.
As for an antenna, a 5/8 wave for the 2 meter band with a magnetic mount will work fine. In hilly areas, the higher radiation angle of a 1/4 wave antenna may work better in some cases. There are many other ways to mount an antenna to your car permanently, if you wish — trunk lip mount, rain gutter mount (do any cars even have gutters anymore?) or even directly to your roof or trunk lid if you’re bold enough to drill a hole through it. But a magnetic mount will work fine, and can easily be transferred from car to car.
For better performance, you could consider using a larger antenna, such as a J-pole, on a mast that you set up temporarily wherever you are stationed. This will be the subject of a future post.