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For lenses many Nikon shooters seem to like: Tokina 10-17mm for WA, the Sigma 17-70mm for general shooting....
Searcaigh, curious why you would choose the D90 over the D7000? Even most experts are putting it above the D300.
Reality is both brands are excellent. No offense but we photographers tend to be the limiting factor, lol. As mentioned Nikon has been "known" for their lenses and Canon for more bodies. You can't go wrong with either.
What you have to ask yourself is do you shoot more land or underwater shots? Many things important to dryland photogs are of less importance underwater, frames per seconds, ergonomics, weight, etc.... Now throw into that how important is shooting HD video to you? Given your budget you are looking at a "prosumer" (sp) body. Pro bodies are three to five times your budget.
Right now the Nikon the D7000 is in your budget and a lot of camera for the money. I just bought one to replace my D200. But like all tech something "better" is always coming out.
For lenses many Nikon shooters seem to like: Tokina 10-17mm for WA, the Sigma 17-70mm for general shooting, the Nikon 60mm & 105mm for macro. Of course there are many other great lenses with their advocates.
Strobes like Sea & Sea, Inon, and Ikelite are tried and true. Again really can't go wrong with any of them. IMHO buy the most powerful strobes you can afford.