Under 55/head, and "Chicago-ish" is screaming pizza / beef / grilled snausage to me. I'd stick to that, OR, do Greektown, but a pizza joint private party should be well within your range, and not overstress the kitchen so the food will suffer. I totally know what you're talking about there - for 100 ppl, it would be pretty hard on about any joint to keep up, and/or put out a decent product. I'm just picturing the whole hotel pans full of beef/gravy, grilled snausages, cold salad, roasted peppers, giardiniera, french bread, and piping hot pizzas being brought out straight from the oven without too much loss of quality for 100 ppl. I bet you could get beer/wine/soft drinks included. For your out of towners, this would probably be a good crowd pleaser - and would probably make the whole "this person doesn't like Mexican, this person doesn't like x" debate moot.
What would you do at La Pasadita (if you don't mind me asking?)
I could see trays of enchiladas, barbacoa, rice and beans, but surely the asada would suffer, no?
Might I also suggest a different turn? A favorite of mine for a tad "out of the ordinary" for a private party would be Reza's. Persian. Private room. Shish ka bobs, hummus, baba ghannouj, pita. VERY mildly spiced. Mostly salt pepper, lemon juice and garlic. A lot of 'newbs' might think of Persian food as something modeled after Indian food, and will be pleasantly surprised. It's tiny bit exotic for some, but their catering packages have things that everyone will love, and will probably make the skeptics very happy with themselves for trying something new, exotic, and loving it.
Looks like you can add booze, and be under your budget too:
http://www.rezasrestaurant.com/banquet.asp
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