Re: best fish finder for 200 to 400 dollars
Since you are mounting on bow troller, assume you are mostly shallow water fishing. Stay with a name brand Eagle/Lowrance, Humminbird, Garmin, etc and find the best screen you like.
Your choices will come down to a smaller color screen, VS a larger black/white grayscale screen. Vertical resolution should not be an issue for shallow water fishing, but having a wider screen provides more history, which I prefer. A larger screen also allows for using a "big digits" feature for depth, which is also important if you are reading it from a standing position on a casting deck. ie you are glancing down not far from your feet.
Make sure the one you choose has a trolling motor mount or can exchange for one. You might have some misc cost for a mount depending on trolling motor and skeg.
If you have a digital or infinite speed control trolling motor, they tend to produce more electrical noise. Try to find someone that has your model of motor and FF you are considering, to confirm compatability. Noise can also be a factor for how it is wired into the system. Read manual carefully, and doesn't hurt to call tech support for recommended appoaches.