Foster parents are generally the first ones to have the opportunity to adopt a foster child of theirs that becomes legally free for adoption. What this means is that the kids who are TPR'ed or low legal risk who are available for straight adoption by an adopt-only family are usually the children whose foster parents said "no." Though in some cases that's just because of financial reasons, or the foster parent being older, or the foster parent really only wanting to be a temporary foster home - In a lot of cases it's because the foster parent never really bonded with the child or because the child has special needs. This means that the kids who are available for straight adoption are, on average, much more "high needs" and "harder to place" than your average foster child and your home will be at least their second (if not 15th) placement which means most of these kids will have some level of attachment issues which can sometimes be quite severe. For this reason, in both states where I've lived (one of which had a true fost-adopt designation and one of which only has "foster parent" or "adoption of legally free child" categories) when we expressed an interest in adopting an individual, younger child with mild to moderate special needs we were encouraged to become plain old foster parents. We were told our chances at adopting a child in that category were better as a foster parent. Everything I've seen has reinforced that impression, actually, though we ended up accepting a placement that fell totally outside the category we thought we were limiting ourselves to! We are moving towards adoption with our two boys. They are our first foster placement ever, and it is their first foster placement. We impulsively said "yes" when we were called about them - They came to us as a 9 y/o and a 1 y/o... siblings, both with special needs. It was totally not what we expected but we're so glad we said yes to them! I'm also so glad we're our boys' first and hopefully last placement.
In my state doing adoption-only means waiting for years if you want a child under the age of 5 who doesn't have significant medical needs and isn't part of a sibling set. I wouldn't imagine that there are tons of low-needs 2-sibling sets, either, but the more siblings and more special needs you'll accept the sooner you can get matched. If waiting longer for a placement that is legally-free makes sense for your family, go for it. But depending where you live, it could take years. Being more open in terms of gender would certainly help you - Can you take a sibling set with one boy and one girl?
Keep in mind that moves are VERY rough for foster and former foster children who've already experienced a lot of turmoil. Even if you're able to legally adopt before 2013 (which is not very likely if you take a placement that's not already legally free), depending on the child's needs they may need contact with birth family such as biological siblings in other homes in your current state, or they may have emotional needs that make a move traumatic for them. I don't say this to discourage you, just something to think about.
Would you consider out-of-state waiting childreN/ I have a friend who is a homefinder for the non-profit Adopt America Network which places only waiting children. Because you're open to more than one child, and potentially to some special needs, it might be worth speaking with them. Not all of the kids they are looking to place have severe special needs, especially if they're part of a sibling set. They also have kids who have medical but not behavioral/emotional issues. Just a thought. You can also look into AdoptUSKids but that tends to be less fruitful than working with a non-profit like AAN.