In order to help you answer this, we need to know quite a bit about what you want to do. There are three questions that will start the conversation:
1 What are you trying to accomplish? Are you finding, inspecting, retrieving or measuring underwater objects?
2)What are your water conditions? How deep are you going, how fast are the currents, and how clear is the water?
3) How, where, and how often will you use the unit? Are you deploying from shore, a boat, or a van?
Of course you want to talk to ROV manufacturers and their dealers. But you should also talk to others who do the same kinds of work you do. Read bulletin boards like this one and http://www.rovinfo.com/ and post a question if you cant find the answer. If possible, see a demonstration of the kinds of tasks you do - in the conditions you operate in. Remember that every ROV looks good on paper and in a swimming pool.