In my experience you only "need" an inexpensive sonar because all you really need to identify is depth and structure, and occasionally it's useful to ID schools of bait or potential fish. And, if you troll frequently, a GPS enabled sonar is nice, but not critical.
I've been using the bottom line Lowrance for about 9 years. It's lasted through many, many trips on the ocean and just as many trips in fresh water. Color screens, side scanning, etc are fun and if you can afford them go for it! But in terms of "need," even the most basic sonars will get the job done.
So based on that, here's what I'd recommend if cost is an issue: http://www.amazon.com/Lowrance-000-1144 ... B00MGAEU0S
If you want an inexpensive one with GPS, there's this version: http://www.westmarine.com/buy/lowrance- ... aQodEk0O9A