It would be best for the agent to drive a car consistent with the cars their clients drive.  Too fancy in a mid priced area and the sellers/buyers will think that you are doing too well.

Too average for an upscale area and the clients will feel you are overstepping yourself.

In either area the car needs to be clean inside and out

I have driven a Lexus since I have been in the business, and works well for my clients.