Is he a taxi company or a town car provider?
If he is a town car provider serving higher end clients, I would go with a Town-car or DTS. The new Lucerne might fit in here as well.
If he is a taxi Company I would go with the Chevy Impala and the mighty Series 3 3800. I also like mini-van taxis as they are spacious and comfy with plenty of luggage space.
While the 300 is nice, I'm not sure how durable it is past the 150K mile mark as it was not designed for fleet usage.