I'm sure you'll have lots of opinions....here's mine with a budget
setup and in some vague order of priority..
Some form of charts
....... paper, chartplotter, pc or phone
based. The medium doesn't really matter.
transponder....yes, this really is worth it. A vhf
radio/ AIS transponder combo is a nice little unit to have on board.
After that I think that we get into nice to have territory...radar, boat