If I'm not mistaken, DealerOn is in the process of rolling out their newer templates so you'll want to consider this when when choosing. I would also call your assigned rep and ask them for examples of their favorites.
As Joe has pointed out, VDP and SRM are your 2 most/more important pages to consider. Be sure your homepage has direct access / quick search into the inventory. I also consider the design and flow of the Specials pages as these are also top viewed pages. Of course just having specials listed is more than what most dealer have.
Randy, will you be incorporating and using their WordPress blogging feature?