I homeschool my 5 children. Some of them are developmentally a little different, but they're actually gifted academically, so it's different than what you're dealing with.
Nevertheless, I think the best way to choose curriculum is probably to start by doing a little work to discover what your philosophy of education is. After you decide how you think children learn, you can narrow down your choices to just the programs that teach the way you want to teach, and then you can make your choice from those selections. You probably won't need special books just because she's lagging behind. You'll just start her at her level, regardless of what grade is written on the cover of the book. You take as long as you need to progress to the next level. Sometimes it will go faster, and other times it will go slower. Sometimes she'll need more practice in an area than your curriculum provides, so you'll get some supplemental activities.
Longterm, you'll have two choices. You can either teach her until she's achieved what you would consider a 12th grade education no matter how long that takes, or you can school her until she's 18 and be happy with what she's learned. Even if she only has, for example, a 9th grade education at that point, it will be better for her to have a thorough understanding of all the stuff people learn in the 9th grade instead of just sitting through more advanced classes and not learning most of the material because it was going too fast for her.
I have two favorite books that I always recommend to people where you are right now. One of them is So You're Thinking About Homeschooling by Lisa Whelchel (that's Blair from Facts of Life). It's an awesome book because she profiles several different homeschooling families and gives details about why they're homeschooling, how their children learn, etc. The greatest part is that she tells you the name of the curriculum they use. You can find the family that's closest to you and check out that program. The second book I'd recommend is Cathy Duffy's 101 Top Picks. It's wonderful because the first part of the book has a quiz where you find out what kind of program is best for you. Then she reviews a lot of available programs for all different kinds of learners.
You're not going to find the "perfect" curriculum the first year. You'll probably never find it, but you will get close. You'll just have to find one to start with that sounds good and looks good. Once you've used it for a while, if it's not working as well as you need it to, you'll have something to compare. You can ask other homeschoolers what they've found that's more of this and less of that than the specific curriculum you're using.
It's a great time of year to consider homeschooling. The convention season is starting, and there's probably a convention near you sometime soon. See if you can find one so that you can go actually look at and touch the books that sound best to you to see if they're as good as you hoped. There will also be lectures you can attend. The lectures that draw the most people are the ones about schooling children with special needs and the ones about schooling gifted children, so you can always count on their being several sessions that address special needs students.