CONGRATS!! I got married in November at the Pilgrim Chapel a few years ago - it's not on the Plaza, it's near Crown Center in Hyde Park - you can look it up online. It was so pretty, but it's for a small wedding and it would solve your church situation. I had my reception in a room at Union Station, but if you did something there, you would take advantage of their incredible Christmas decorations.
You could also look into some of the hotels that sit over on Ward Parkway - The Rapheal for instance.
If you go to the knot.com, they will have a link or a portion of the website that you can specify Kansas City and it will link you to countless options for reception locations, with cost estimates and contact information.
I would have poinsettas everywhere if were me, with gold ribbons. Also, I think using those red-berry-type things in your floral arrangements would be pretty and festive. You could give small stockings with candy as a favor or if you have more to spend or it's a small wedding, ornaments so people will always have that to remember your day. Your food could also be Christmas-themed - cranberries, cookies, that kind of stuff. And definitely lights, I would have white Christmas lights everywhere. And, finally, you could put mistletoe around your reception, that might be cute and romantic.
Good luck and have fun!!