Hi,
I would do a lot of homework before deciding.
You will pay a lot for the displays, radar and other options, so I wouldn't get hung up on cable and transducer re-use.
Check out all the leading brands. Talk to as many people as you can. Go and use them all in the same place and lighting conditions. Do you have any experience with a particular brand? How far are you from their service centers if you need it? Determine what you want: multi function display(s) / size, radar, radar overlay (this usually requires a heading sensor and maybe attach unit), radio, weather, thruhull ducer or shoot thru, map card, antenna, gps sensor etc?
Then check out the sizes of the displays and other units. Do they fit in your locations on your boat? Do you have to do something custom to make them all fit?
Once you lock things in, then start pricing the gear. Definately use the net. BOE Marine, Consumers Marine, do a google search and price at least 5 places. Then price locally (store and maybe dealer/installer). Determine your price bands and then decide if you want to get off the net or buy locally. Check installation options, dealer or maybe a local installer or DIY. I found the best all around was to have a reputable local installer do the work to your specs and buy everything off the net. Usually you can get no shipping and no tax on top of a low price. People will say buy locally, but IMHO this kind of commodity purchase comes heavily down to price. The gear is the same at the end of the day. If a new boat - work it after you settle on the boat price. Usually you can get a good deal on installation.