I have a 10yr old Lowrance with a 7" screen, that was pretty good for its time. It doesn't have side-scan or any of the new technologies. I am a casual fisherman as well. I do use split-screen often but am mostly interested in seeing the depth, contour lines, water temp, and bottom composition (soft versus hard bottom to find the transitions). If that is all that you need, you should be able to find a good, used unit for your price range. If you're going to struggle seeing a "smaller" screen, then plan on spending more money.
I don't know if it's the same, but in the past Hummingbird linked with Minnkota trolling motors and Lowrance linked with Motorguide. If that is the same, make sure to get a graph that can work the contour lines with your trolling motor.