I have sailed a Dart 18 for several years, I am sure a gennaker will be a good investment.
Here in Europe I have seen several Darts with a gennaker/Spi
If the setup is right you can even handle it by yourself (solo).
For a long time I did want to buy a gennaker kit myself, instead I bought a N 6.0 (with Spi) because of the crew overall weight. I still regret the day I sold my Dart 18 It is a great boat to sail Solo.