Hi K. :)
If you need any help discussing the ups and downs of foster care, please don't hesitate to PM me! I have been a foster parent thru the state/county for 3 and a half years and have learned a lot in that time!
I don't have any knowledge of fostering through a private agency. I can tell you though, that if a child is in foster care, there is no guarantee, no matter which route you choose, that you will get to adopt. I have had six placements and adopted one (just got word yesterday that we are moving to adoptions for our foster son, though! Soooo excited to be able to say I have two kids that are mine!).
Keep in mind that the goal is ALWAYS reunification. Parents are given every opportunity to get their children back. Sometimes they do what is needed and their children get to go back home. This is a good thing! Sometimes the parents won't do what is needed, or simply can't due to mental health issues or developmental issues, and if that is the case, after a certain amount of time, the courts will grant custody permanantly removed from the parents. DCFS will then be responsible for finding the children a forever home. Please note that you will NOT be next in line at this point. Even though this child has lived with you and you have put in all the hard work, you will not be next in line. DCFS will explore every option they can find to place the child within their birth family. If there is a grandparent, aunt, cousin, etc who is interested in taking on the responsibility of the child, they will be given the option if they are a stable environment. Once those options are all explored and ruled out, you as the foster parent (if your license is foster/adopt) will be given the opportunity.
It can be a super long process, but so worth it in the end!