You said GPS, does that mean mapping? 1' contours and such.
If a base screen is acceptable, look at the striker series by Garmin. Same transducers and pixel count, as the higher end units. Unless they've changed things.
I agree 100% with above statements of "as big as you can afford", and "call Brian at bbg". He's quite knowledgeable.
SI is good, as it can help too find things off to the side, then switch too DI to get an excellent view of the object. However, what you see is a combination of your eyes, how much time you've put into to adjusting the settings for that days conditions, the body of water, and the screen size.