Welcome!
I have had LOTS of babies
so I have been there for sure. Good for you for thinking about this already - with my first child, I was pregnant again before I knew it and still had 15 pounds to lose (which I have still not lost)!
A few things I have learned over the years:
1. Be sure and give yourself time . . . it always takes me at least a year to lose my pregnancy weight. My last baby was born April 16, 2005 and I am finally within a couple of pounds of pre-preg weight (with that one - still have weight to lose from the others!). Of course, that could have something to do with the fact that I gain 50 pounds each time
2. You are smart to be worried about nursing - some women can really diet big time while nursing, and others notice their milk production dropping almost immediately.
3. I never lost a whole lot of weight until I stopped breastfeeding . . . maybe just me but I tend to hang on to 10 pounds or so at least while nursing.
4. You have lots of options as far as diet goes. The two major things I think are that you need to avoid ketosis (ketones are excreted in breast milk and while never proven harmful to the baby, why take the risk?) and also avoid cutting your calories drastically or your milk supply may be affected.
My personal opinion is that the best diet for nursing mothers (and mothers in general) is the South Beach Diet. My husband is an OB/GYN and he agrees - in fact he has no problem with recommending people follow it while pregnant (Phase 2-3), as he says it is very similar to the diabetic diet they put people on with gestational diabeties. He also says is basically is just healthy eating - no refined carbs, lots of veggies and lean meats, and some whole grains - and he thinks we should all eat that way anyway.
The good thing about the SBD while nursing is that you really do not cut your calories (except by cutting out the bad stuff), so you can lose weight while keeping up your milk supply.
Having said that, my sister (also an OB/GYN) just had her third child the end of May. She has started "somersizing" which she did during her second pregnancy to great success. She wanted to do something a little less "depriving" (not as healthy though I don't think!) where she could still have full fat cheese, cream, etc.
Those are the two diets I am most familar with being combined with nursing - I am sure there are lots of others!