The weight fears are totally natural... and they're only a bad thing if the fears keep you from gaining the weight the baby needs. Maybe look up in a pregnancy book the breakdown of all the weight gain. One of the early things that causes weight gain is blood volume since our volume starts to go up. It may also be bloat/constipation at this stage so don't worry that it will be there forever.
If you're slim now and good with your exercise and stay in shape through your pregnancy then you will have an easier birth and recovery and find it easier to get back into shape (a few weeks off here and there for medical reasons are not going to make you totally out of shape).
As for weight loss with breastfeeding, my mom breastfed all 3 of us kids, and she said I was the only one she lost weight with... that I was just a hungrier baby. You have to keep your calories and fluid intake up while breastfeeding or else you may dry up your supply and should probably be especially careful about that in the first few weeks as the milk supply gets established. Find a lactation consultant who can help!
I do have to laugh that I'm thrilled to finally have an excuse for my stretchmarks... my boobs grew so fast in high school that they got covered in them... I finally have a reason! I have a few just from being fat too that I'll be happy to claim were caused by this baby

They do have that new Mederma cream that is supposed to diminish the signs of stretch marks (its 'clinically proven' according to the commercials

) so at least if you get them, that may help speed it along.
I do worry about the deflated boobs.. mine haven't really been perky (at a certain point gravity wins when they're this big) but they have been full and not flat. Oh well though, not much I can do about it.