I'd get the same make of radar as your chartplotter. Partly because the
knobology will be the same between the two instruments, and partly because
they will talk to each other better. I have a Raymarine chartplotter and
bought a Furuno radar, and they won't properly share NMEA data - both
manufacturers blame the other half for the incompatibility. I bought Furuno
because all my research showed that they are fabulously reliable (and my
personal experience has confirmed this) but I still wish I'd bought
Raymarine because of the knobology and compatibility issues.

I have a sailing boat, and I have my radar inside. It would be much more
useful on the outside, but the downside is that it would be exposed to sun,
salt and getting wacked by ropes etc. I have on my list of things to do to
make a swinging arm that in is parked position holds the radar on the inside
above the chart-table, but on its swung position holds the radar in the
companionway where I could see it from the helm.

Stick the radar halfway up the mast. If you have a rear gantry it is better
there, especially if gimballed. The nominal range of the smaller Furuno,
Raymarine and JRC radars is about 12 miles but in practise they are only
really reliable over about 5 miles. They will pick out land over a greater
distance, but that is not really helpful. Ships are the danger, and they are
only really visible at about 4-5 miles.