Re: Speed/Depth/Wind conundrum
I think it's worth thinking through your long term plans before buying, rather than adding to the system constantly. I'm building out a new electronics package for my boat, and one of my goals was to have fewer display units, not more. The plotter (I'm using a Raymarine e7d) already has a nice big display and I didn't want to duplicate that into more instruments. I don't want a huge dashboard covered with lights and numbers blocking my view.
If you buy two or three display units now, then add a chart plotter later, you are going to end up with a lot of electronics to put into a small cockpit.
I'm building my own NMEA display unit that will hang inside my dodger and have built my own pitch and roll and wind sensors (that is using a Davis windspeed/direction sensor, I built the bits to convert that to NMEA). Building your own isn't the most logical option (and likely isn't an option at all for most sailor), but it's been a fun project for me. My display and the e7d will be the only things visible in the cockpit.
I sometimes get annoyed at the lack of real integration in boat electronics. I'm going to add a wheelpilot this spring and it's annoying that the Raymarine wheel pilots require a P70 (or similar) user interface head even when there is a fancy touchscreen GPS sitting right there that could provide all of the UI.
As for what's useful I find it useful to have speed over ground, speed through water, and depth. If you are on a lake then speed through water isn't so useful. On my old boat (mostly sailed on a lake) I got good enough electronics using a handheld GPS on a fixed mount and a fish finder for depth. The total outlay was a couple hundred dollars, I could read it from anywhere in the cockpit, and no holes were drilled in the boat.
1986 Pearson 28-2 "Elena"