I was looking at the raymarine i40, good price, but it turns out it’s not meant for sailboats
Not sure about that statement. It does come with 65' of cable for mounting the wind transducer on the mast. It's appears to just be a low cost alternative to the i50, i60 and i70s packages. The display is more basic and it only works with the Rotavecta wind transducer. Being $800 less than i50/i60 and i70s packages if all you want to know is wind speed and direction AND you never plan to build an instrument network, it is an option.
The other instrument/wind packages on Defender start around $1250 for Raymarine and $1600 for B&G. That's a hunk of change to put out but they do get you started with a NMEA 2000 (or SeaTalkng which is Raymarine's version) network. From there you can add on future instruments and transducers to share on all displays.
Are wind instruments necessary? In my opinion not really, for the same reasons others have stated.
My priority for instruments would be depth, autopilot, chart plotter, then wind. When I was building my system I planned ahead for installing a SeaTalkng system as I had already decided to go with Raymarine instruments.
If you decide to go with a wind package, a very nice package I'd suggest is the RM i70S display with depth and wind package for $1250. The i70s can display wind, depth, GPS speed, speed from a knot meter, heading/bearing and more. It has multiple screens you easily set up to show what you them to show.
The iTC5 converts transducer signals and puts them on the SeaTalkng network making them available to any connected display. No need for dedicated displays. You probably don't need to install the new depth transducer if you already have one as it may be compatible with the iTC5.
I do agree with
@Scott T-Bird s comments in post #3.
Personally, I mostly day sail and frequently solo. I love being able to set the AP to "Steer to the Wind" on days when I just want to relax on the water.