If you are reasonably handy with basic power tools it's not a big job to install one. The toughest part of the job typically is running the transducer wire from the transom/bilge area up to the dash. GW includes a PVC pipe or two under the floor and you will have to use a plumbers snake to get through it to pull the wire through.
As far as connections, you will have a power and ground lead - both will connect to your fuse block (+) and (-). If your VHF is newer and supports NMEA 0183 then it pays to connect it to the new GPS/FF unit so that your position is captured by the VHF in the event you ever have to use the distress feature.
Go for the biggest screen your wallet can support.....there are good deals out there right now on the Garmin 740s (discontinued but still available) and the Lowrance HDS-7 (Gen2) or Simrad NSS7. You should be able to get a decent 7 inch screen unit with transducer in the $850-$900 range, if not less.