That is certainly a good approach if you already own a PND. However, I would be more likely to investigate laptop mounts for your vehicle and computer. For rallies, I would think the larger screen would be a great asset. If you plan it carefully, the hassle of installing/ removing the laptop can be minimized and the advantage over a tiny, low pixel display will make it worthwhile.
If you have your heart set on a PND, save the hassle and move to a program that supports it.
Terry