I'm starting to look at some SI units as I have never owned one before. So far I like what I've seen and read about the Humingbird 999 SI. I fish a big river system a lot and can see where this would help me tremendously in locating the constantly changing structure. Any advice about this or other units I should look at would be appreciated. I'm not in a hurry to purchase and would like to find a deal on a new or possibly slightly used unit. Thanks in advance for your help.