You have a lot of choices to make. Color/Monochrome? GPS/No GPS? Display size? Do you have a total price in mind?

If you could post what is already installed, that could help with us recommending units that might use the same wiring and transducer. Also some idea of what lakes or what kind of water you fish will help as well.

A guy I fish with runs Lowrance X135's, and they are nice for a reasonably priced unit, but I think color is worth the extra money, and that GPS with mapping is indispensable on many lakes.