Fishfinder question for the pros

[Liveaboard]

Hey everyone,
I'm in the market for a good fishfinder but I am in the dark as to what I need. My boat is a 43' Viking, my wife and I live aboard so it is an aft cabin boat and not a sportfish model, all the same we both love to fish and get out whenever we can. All that we have aboard are two Datamarine depth finders that don't work so well when the engines are running. The depths displayed are all over the place and there are noise filters mounted on the engines so I don't know what is up with that. What I would REALLY like to have is a good fish finder for depths up to 600'. We like to bottom fish for cod and haddock but we also enjoy the striper fishing. So, do I buy new or used ? Can I get a good quality fishfinder that is a transom mount, cause I really don't want to put this thing on the hard to install it? What does through hull mean, does a through hull actually transmit the signal through the hull from the inside or does it require drilling through the hull to mount the transducer. Can somebody steer me in the right direction here? Thanks for your help

[Deep C]

We have a 43 express and have a middle of the road furuno fcv585 that came with the boat. Ot had a kind of low end transducer that we are now upgrading to top shelf. Very good unit. As a back up we have furuno 1650wf which is also a gps plotter... We just moved the old transducer wire to it...