I also need one for the front of my boat to use on the TM which has the built in US2 transducer so I am looking at a GPS/sonar combo. The HB Helix seems to be the best deal going. It has a 5" screen and color too for $300. I have a HB385 on it now and after using my HDS7 for a while, the screen on the HB looks really small.
I keep thinking one of the members here is going to upgrade and post a 2D/GPS combo for sale.
By the way, I would suggest getting the same brand as you have on the console. It is easier than having to switch back and forth because each brand has their own peculiarities. I know because I have a Lowrance and a HB and I have to remember which button does what.