I'm looking to buy an over/under primarily for pheasant hunting in Illinois. Any ideas, suggestions on brand, chokes, etc.

I would suggest looking at Beretta and Browning, they both make a great O/U's. They are going to feel and fit you differently, I would suggest you go to a shop and try as many out as you can. As far as barrel lengths are concerned a 28" is a good compromise. Give a modified a try with 1 1/4oz or 1 3/8oz of #5's and call it good.
Nothing set in stone, your milage will vary, my $.02

If your using a PointingDog then a 20 -28 gauge is all you need I/M choke will work well.

If your foot hunting with a Flusher then a 12 gauge with 6's is all you need with I/M

I have a 5 3/4 lb Ruger o/u 28 gauge which no one is going to pry me from and a Beretta 12 gauge autoloader one is for the pointer and the other is for the flusher.

I like the Ruger Red Label 20ga with 26" barrels shooting 1-1/8oz #6's in IC and Mod.
The best thing you can do is go to a gun store and try out all the different brands to see which gun fits you best. Then work up a load from there.
Good luck.

I'm partial to the Spartan 310. Well built, but cheap enough to take hunting without guilt.

I like a modified choke in barrel #1 and a full (presumably for further distance on the follow up) in barrel #2.

As far as brand of O/U, I have owned several and prefer a Beretta 686 for the money. I would go with a 12 ga.

Another vote for Beretta and 12 ga.