It might help if you tell us where you'll be using the equipment(inshore, nearshore, offshore) and what size of boat the equipment is going on...
For example, I fish inshore from a 17' boat - I've got a Humminbird depth/fish finder and a Garmin GPS. The Humminbird communicates with the GPS so I really only look at one screen,but, if for whatever reasonthe depth finder goes out then I can still use my Garmin to get me home. My setup only cost me about $600 or so.
But, my buddy's boatis a 24' offshore boat- he's gota Raymarine chartplotter/GPS/Radar thingee that costs a few thousand.
The point being, where you go and what you're going to be doing will dictate what kind of setup you need.