I can't speak to the switch.
An 18" dome gives less detail - a bigger smear - than a 24. The wider the antenna
, the smaller the angle of the beam, ergo, more detail.
KW give you range. 2.5 is minimum, 4 is better, basically seeing something big at long range. 2.5 will run out at about 16 miles and 4 at about 24.
If you are replacing the whole thing, consider the Furuno
or Garmin wireless domes. I combined a Fununo with a refurbished iPad
as a receiver for under $1,000, and like it. It's easy to install, because all you need up the mast
is a honkin' big 12v DC pair.
Congrats on the new boat
and good luck with the project
All of this is bottom end radar
. On your boat
it's fine. You're not going to hit it any time soon, so you'll probably use the longer ranges only to keep an eye on the shoreline.
Learn to use it by running it in full sunlight, comparing what you see in the radar with what you see with your Mk I eyeball.