Hello there. Unless I have misunderstood, you have posted this in the London questions. Perhaps posting it in the relevant African forum sections might be better for you as you are more likely to get answers from people who know about that place.
Will you travel with your own car?
The route from nairobi to Arusha and from Aursha to dar Es Salaam is completely paved and in fair conditions. No much traffic. great landscapes and interesting villages on the road.
You'll cross the border in a couple of hours provided ther is no trouble.