It is a very difficult decision to put your child on ADHD medication, and the fact that you realize this tells me that you have put a lot of thought into making this decision. As a child therapist who has worked with a lot of families with ADHD children, I have a few words of advice. First, as long as you have multiple sources (school, daycare, other parents) saying they see the same behavior problems (of inattention/hyperactivity) in your daughter as you do your diagnosis is probably right. Getting a good diagnosis from the right person is very important. There are too many pediatricians in the area that I live that think they can diagnose these problems and medicate children. I end up seeing them years later when emotional damage has been done due to misdiagnosis, parents not getting enough support/help to deal, etc. I would make sure that the school, doctor, therapists, etc are all on the same page. ADHD symptoms can actually be something other than ADHD, such as allergies, other health problems, or another type of mental illness. ADHD symptoms can stem from a child who does not have enough structure in their life. Once the diagnosis is established and medication is begun it can be a while to get the correct dosage and type of medication, so be patient. You really need your daughter on your team if you want this to be effective. It takes a little bit of time and patience, but should be done from the beginning if possible. I always talk to the children in a way they can understand. Use the analogy of eye glasses- some people have bad eyesight, it is not thier fault, but what happens if they resist getting the help they need (headaches, can't see) and relate that to ADHD. Explain that you can't see the problem but some of the symptoms are...Then explain that medication is like eye glasses, it helps you to focus, not run into things, and even listen better (whatever your daughter's problems are).
As far as your guilt, if you have covered all of your bases you don't need to feel guilty, but maybe there is a reason you do, I would really think through what is going on inside your head. Maybe you feel like you are rushing into it. Maybe you are feeling pressured into this. Maybe it is guilt related to something that happened to you as a child, that this event somehow reminded you of. Only you can tell. But figure out why you feel guilty about this then address it.
ADHD is a true illness and medication can be powerfully helpful. Science is getting better at diagnosing, there is now a brain scan that shows electrical brain activity that can be used to diagnose ADHD and show what type of ADHD it is. It is amazing, but not available in all areas and can be expensive. The amazing thing about it is once a person with ADHD is on medication, the brain scan has then shows normal brain activity!
I personally have seen amazing differences in children who really needed the medication and were put on it. It has changed their life for the better. For some ADHD kids EVERYTHING can be a struggle, but medication finally brings peace because they can focus. I have a three year old with ADHD symptoms, and someday soon may have to make a choice whether to medicate or not. I will make the choice based on her, if she is indeed struggling at school, making friends, etc, I know that it would do more to help her than hurt her. I want her to feel confident in herself and capable of doing the school work, because I know that she is smart. But I also know that if it is difficult for her to focus and sit still, she might not be able to apply herself. I can manage her behavior at home (though it takes much patience), but I realize that school may be different.
One last peace of advice, parenting an ADHD child is extremely challenging! Medication is usually only effective while a child is at school (stimulants need to get out of the system by bedtime), so it is important for you and your husband to begin learning now ways to help a child with the specific struggles that your daughter has. There are many books, internet groups etc out there to help support you and don't rule out a child/family therapist. Know that you are not alone!