I think the decision of what lens(es) to invest in is far more important than what body you are going to go with.
Either D70 or D300 will work. Yes the D300 is sexier machine, but I guarantee noone will look at your images and say "those were shot with a D70".
However, you will not be happy with trying to shoot with the 18-70mm lens. Better decide if you want to start out shooting macro or wide angle and then get the appropriate glass. Having the right glass WILL make a big difference. And that WILL be noticable in your images.
Decide on the selection of strobes AFTER you make the decision on if you want to start out shoooting Macro or widenangle. The DS125 is a great stobe. You might want to start with one if you are shooting macro and add another one later is you go on to shoot WA. Ir you might go with two smaller stobes for macro.
IMHO the camera body is the least important part of the puzzle. The most important part is YOU. First decise what you are interesting in learning to shoot. Then consider what glass you need. Thereafter consider the appropriate strobe setup. If money is a consideration, at this time caluclate what it all will cost you and if you still have the $ to spring for the cooler body/housing combo, my all means go for it. But don't worry about that last bit too much. The fish won't care. The editors don't care what body you shot that great images with. But if you don't have the appropriate glass... well the images just won't happen.
Take you time, think about it and you'll end up with a kit that you will forever fondly remeber as your "firstborn" 
