Can't help you with Ill. law. I don't think you need anything more than what you have. There may be a limit on how much BP you can keep in your house, subdivision, county etc.
What are your interests? Cowboy Action, Civil War, Buckskinner, Rev. War, `1812??
Most economical is to look local for some things and others buy on line. Over all, if you are with a group of the same interest, good used gear and equipment can be reasonably had. Group buys can be arranged for powder and cap purchases on line for much less than you can usually buy local.
Where are you in Ill? If you are in the Springfield area, check with the 114th Ill. Vol. INF Regiment at Lincoln'sTomb. They are involved in many eras of the BP shooting back to Rev. War forward. Principally, they serve as an Honor Guard for the Tomb and I believe ?? a ceremonial roll for the Governor also. They have some that re-enact, competition shooting, and ceremonial duties as well. You can pick your favorite if you can get in to it.
Addition:
Where you are in Ill. matters also in getting your gear. North different places, South end Dixie Gunworks in Union City TN is worth a trip. At least get DG 's catalog as a good reference guide to BP.