IMHO, if you want a chartplotter, these are some features you should consider:
- color screen (not required but it's a lot easier to see a red nun and green can when the colors are displayed on the screen
- big screen - the bigger the better. IMHO, 3.5" MINIMUM, with 5" being better and 7 or 9" even better
- charts that can be updated
- easy to network to other instruments
I don't know if you can get all that for $400, but it won't be much more.
Some good brands include Garmin, Lowrance, Standard Horizon, Eagle, Humminbird, and a few others.
While you may not NEED a chartplotter, they certainly come in handy. You can plan a route when you tied to the dock or mooring, and then navigate it safely when weather gets bad. A novice can easily steer the boat when you get an arrow pointing the way. You get a lot of good information, like Estimated Time of Arrival, Velocity Made Good, Distance to Waypoint, Time of High Tide, Predicted Tide, plus all sorts of Points of Interests like marinas, fuel docks, etc.
Lastly, if you think you might want to connect other instruments to your plotter, like VHF radio with DSC, autopilot, etc, then look for a unit with NMEA 2000 interfacing.