I am really sorry to hear this. I did a little research at Illinois law and there was a case in 2011 where 2 children attempted to sue their mother but it was tossed out by the district court and also dismissed in the appellate court. Illinois courts are very reluctant to get involved in parent/child disputes. However, they did say in their opinion if the conduct was excessive and outrageous, they might have considered it. I would suggest contacting a local attorney for help investigating whether your parents conduct would meet the excessive or outrageousness test that Illinois courts apply. I will try to answer your questions presuming you do intend to follow through...
My answers are in bold faced.
1) What evidence do I need to make my case? I've taken meticulous notes regarding the abuse, some of which I've emailed to my parents but received no reply from them.Your testimony will be key but any photographs of injuries or any text or written communications that were abusive would help the case.
2) Will I need to live in Illinois at my own expense during the proceedings? No, not necessarily but you may need to fly in for hearings or depositions if there are any.
3) Will my therapists need to testify (again, at my expense)? They can, and yes it would be at your expense.
4) Will I have to hire an Illinois-based attorney, and if so, what category of practice would cover this sort of case? Yes, you would want an Illinois based attorney who specializes in family law disputes.
In addition, I would greatly appreciate any recommendations for books, periodicals, Web sites, or support groups that assist adult survivors of child abuse with obtaining redress through the legal system. I came across this site. http://www.askthejudge.info/should-kids-be-allowed-to-sue-their-parents/