Most agencies are negotiable somewhat on their listed fees. I would recommend not to go with the cheapest option. Perhaps check up on a couple of the property portals and see if any agency has a unit in your complex or close by, that might be a good starting point in your search for an agency.
Make sure that who ever you choose to go with use a contract between you and the agency with what services is included and what is not in their let and manage package. (this is called a letter of engagement and is a requirement under the new property services regulation act)