As
usual, it depends on what kind of photography you practice primarily.
For more or less common usage, a strobe with about 50 W/s should be
more than adequate. While they might be a bit weak for the inside of
a wreck of in caves, they're light and relatively cheap.
Consequently,
we recommend a Sea&Sea-YS50 TTL/N or a YS60 TTL/N (N stands for
Nikonos connector). Our Adapter was originally built for these strobes;
a bit of effort should make it possible to find one for about 120-150
€. For a slightly improved performance, you might want to buy a
YS120 TTL/S or -TTL/N.
However, you should be prepared to pay a noticeably higher price.