unless you're planning on extensive travels, you can easily get away with a depth sounder (mounted inside if you don't want to punch holes in the hull) and a good gps/plotter. the gps will give you speed over the ground and electronic charting, the sounder will tell you when it's getting too thin. the other 'must have', so far as i'm concerned is an autopilot. you can find them used online and, if you singlehand like a lot of us do, you'll find it a lifesaver. you can load up with wind/speed, radar, etc, but these are the basics. of course, a good VHF is a necessity - and i'd go for one that either has or offers a remote mic.