I didnt see anything in the OP that made it seem they "only" wanted to adopt and wouldnt be good candidates to foster, nor did i see anything that would suggest they only wanted a healthy child and wouldnt be prepared if a child had some issues. I think private infant adoption can be a good choice (although, again, didnt see that the OP *only* wanted an infant) but private adoption isnt feasible for everyone.
While i absolutely would say that if you foster you MUST be able to support the plan and must recognize that you might not adopt a child in your care, i dont like the idea that i see in lots of places that fostering is really only suitable for those that would be willing to adopt but do not desire adoption as their primary goal. I think you can be a good foster parent who supports reunification and be willing to work with birthparents AND also really really want to adopt a child permanently. Its hard when you have both of those emotions going on, because they somewhat conflict. But as long as you are aware of them and what the reality of the situation is i think its fine. Obviously if a foster parent was trying to sabotage RU or was being inappropriate in usurping the role of the birthparent thats an issue but it doesnt HAVE to be that way.
To the OP the short answer to your question is that its very likely you will be able to adopt a child you foster, although it might not be the FIRST child you foster. I think its pretty UNlikely that you "never" get to adopt, although that certain can happen because anything is possible.
The long answer is that with fostering there just are no guarantees. And even in cases that everyone thinks will go a certain way, may go a different way at the 11th hour. Depending on where you live and also maybe your particular agency you may be able to increase chances of getting a probable adoptive placement (sometimes they know from the beginning the child is unlikely to go home) but again there just are no guarantees. But can you adopt a young child via fostering? sure, happens all the time. I know tons of parents online who have adopted multiple healthy infants and toddlers within a a few years time via fostering.
My own personal experience is that i adopted my first placement. Healthy 3 week old infant, no birthparent visits, TPR at 4 months, finalized at 11 months. My second placement, a healthy 1 yr old girl, went to an aunt after two months (it was expected), then i adopted my third placement, a 16 month old boy at placement (2 yo at TPR, 3 yo at finalization)...i adopted his 8 yr old sister after TPR as well.
I took a couple years break from fostering and when i started up again i had a sibling group that was RU'd after two months and then i had a 2 yr old that was Ru'd after several months, both of those reunifications were expected and celebrated.
I think its probably a little easier to foster and have a child leave after you have already gotten a "keeper", i would think. I know of people who were desperate to adopt, got a wonderful first placement headed to TPR then the child went to a relative after a year or two and it was devastating. But then they went on to foster more kids and eventually adopt.
Good luck!