If your HB 170 is functioning as a depth finder, unit @ console & transdx on transom ... then something to mount on your bow/trolling motor should work well for you.

You could get something like this unit : Amazon.com : Lowrance 000-11146-001 Elite-5 HDI Combo with Basemap and 50/200-455/800KHz Transducer : Boating Chartplotters : GPS & Navigation ... and have color 2d broadband sonar, down image sonar, and GPS with a basemap (lake map outline without contour depths) and have any one of them, two of them, or all three of them showing on screen at the same time.

The 2d broadband sonar = cone shaped ping signal that gives you a color image like your HB170
The downscan sonar = narrow band ping signal that gives you a color image more like a photo
The GPS/chartplotter = lake outline map, waypoint capability, speed, retraceable routes/trails
The transducer has dual 2d & dual downscan kHz settings & water temp sensor

And if you don't want/need the GPS mapping capabilities ... then something like a Elite 5x HDI would give you the 2d & downscan capabilities for about the same price or less.

I'm sure that Humminbird & Garmin have similar units that are in the same price range, I'm just not familiar with their line-up of comparable units.

I'd figure out "how" (what methods) I'm likely to be fishing most often ... consider what capabilities the unit needs to have to make those methods easier & more productive ... find a few different brands/models that have those capabilities & stay within your budget ... price shop the web, then contact Brian@BBGmarine (site member/sponsor) and talk with him to finalize your choice & see if he can beat the web prices you found.

I wouldn't recommend getting a unit with a screen size less than 5" ... and, in fact, if you didn't want the GPS capabilities, you could get a Elite 7x HDI for around the same money as the 5x HDI. Amazon.com : Lowrance 000-10961-001 Elite-7X Fishfinder with 83/200-455/800 KHz Transom Mount Transducer : Fish Finders : Sports & Outdoors

... cp