If $200 is your budget, I would recommend either the Lowrance Hook 4X HDI / 000-12641-001 or the Garmin Striker 4 DV / 010-01551-00. Both have Sonar/Down vu or down scan (Lowrance) and the Garmin also has a built in GPS with a basic map you can mark waypoints on. You can get a trolling motor kit for the Lowrance for around $19.95 and the Garmin ships with a trolling motor bracket. Either one of these units would do a nice job for you at that price point.
Brian