The E350 V10 makes an awesome tow vehicle. An ambulance also makes a really good tow vehicle and can be found for cheap. It will have high miles on it, but it was most likely taken good care of. They are all over ebay.
All of my brother's buddies run E350's for transporting their bikes around, they call them Motovans. Here is one from ebay:
I'm looking at this one Friday for $10,500