What is their attitude to costs? Will they save us money?
How will they relate to our media buying agency/other specialists?
How important to them will our account be? Will we be one of their larger accounts, or simply a small fish in a very large pool?
Will they fit with our ways of working? Are they willing and able to be business partners, or will they simply be suppliers? (This depends – of course – on how we see our own style of dealing with agencies)