Legally adopting your stepchild is a big step. Doing so offers a lot of benefits, the main one being that you and your stepchild officially become "family." That gives you certain rights to custody and visitation if you and your spouse divorce. It also imposes a support obligation on you for that child's welfare. Your stepchild also gains inheritance rights and the right to important benefits if you should die.
Money disputes can be among the biggest conflicts in a marriage, so a lot of couples are opting to keep separate bank accounts in addition to a "household" account that's jointly owned and used to pay the bills. That way, each spouse retains control over their own money and a certain measure of financial autonomy from the other spouse.
Being the parent of a special needs child can be daunting and lonely. It may often feel like nobody quite understands exactly what struggles you're going through -- nor do they understand your fears for your child's future. After all, children with special needs eventually become adults with special needs.