Buddy, the lower 2/3 of Illinois covers a LOT of different types of water. Be more site specific. If I had to be anywhere near the main channel of the Miss or Illinois rivers, I would say 16'/25hp-MINMUM, definitely bigger as you travel downstream. I'm sure the Carlyle and Rend guys would have recommendations if those lakes are on your agenda.
Personally, I've hunted the Miss, above Quincy, for years with a 16' Jon and 25 hp, and rarely felt in danger. However, I've also paid very close attention to the weather, and there have been times that I went back home without even launching.
Cold water and a swamped boat is often deadly. Your choice of watercraft is no place to skimp. Spend all you can afford and then some.
Semper Fi, Marines
Casey 1997-2011 RIP