"An external flash for Nikon digital" means it would be a TTL or iTTL flash.
Any TTL auto flash will not work well because it will not be able to communicate with the camera. This generally means that it will fire at full power.
A simple manual flash will work, but you will have to do many calculations using the flash power guide number and the distance of the subject to get the correct aperture for the correct exposure.
The best flash to use is an old-fashioned movie-age flash with an automatic thyristor sensor on the front of the flash that will automatically vary the flash power to match the aperture and ISO selected on the camera.
Some Nikon flash units of the previous movie era also used automatic thyristor. Newer Nikons such as the SB-800 and the SB-900 also have an automatic thyristor option.
A good example of an older automatic thyristor flash is the Vivitar 283.
Vivitar 283 user manual