legal guardianship and adoption are different animals. I'm not sure of the legal intricacies involved but if you're going for full adoption, even a consensual adoption can be costly. That being said, there are some advantages to a full adoption.
That being said, legal guardianship can work quite well in many cases. I would find a family law lawyer (hopefully one with adoption/guardianship experience) and discuss it with them as the first step. Otherwise there may be resources in your AO, call around, ask around. There are a lot of charities that deal in this specific thing day in and day out. Even if they can't provide free assistance to you (they are generally there to make sure the kids that are already in a bad situation can get out of it), they will quite often give you advice and walk you through some of the legal mumbo jumbo that you will have to deal with.
IANAL