Start out by going to
AIRMAR MARINE PRODUCTS
Chances are your transducer is manufactured by Airmar. Most of the transducers have a different plug that will fit specific head units. If you buy a different brand than what was originally installed in the boat, you can often change the plug on your existing transducer cable to fit your new unit with some basic wiring and heat shrink tubing.
For $700 bucks, I would think you could get a nice combo unit (gps/fishfinder) with a transducer.
I use Lowrance HDS and am happy so far. I fished a buddies boat this weekend with a 4" Garmin and it was great too. You basically can't go wrong with any of the brands though. The skimmer transducers that most smaller units come with will work fine for most all applications.
Visit West Marine, Bass Pro, etc. to put your hands on some working displays.