Well, what can I say...........Lowrance. Yep, that's about all. There is no doubt that Lowrance are a step ahead of the comp in sounders. For $500 you could pick up a good unit. The higher the pixel count, the clearer the screen and images.
GPS, doesn't sound like you need one. and if you only have a $500 sounder budget, you won't get much change to get a GPS, which start at around $600 for a good sized, good quality unit. Here , I like Garmin. The 126 in particular, light, compact, inbuilt aerial, high satelite tracking, very user friendly.
NO hand helds............that's just my opinion.