You'll be fine and everything will bolt right up. I fitted a K1100RS fairing to a K75. The only issue was the mounts/brackets on the frame near the top of the radiator since the K75 is "missing" a cylinder so those frame members angle back farther. That won't be an issue on a K100.
(http://i82.photobucket.com/albums/j246/smithduck/k75fs.jpg)
That's an old picture. My K75 actually has K1100RS side covers now.
(http://i82.photobucket.com/albums/j246/smithduck/2012/121210_k75f2ca/121210_day2a.jpg)
Like other Ks the K1100RS fairing does get warm in the thigh zone. One thing you can do is add a blanket of insulation under the tank. In my case I added some perforated stainless grills to the lowers which restricts airflow to some extent so it's about a wash in terms of adding the blanket. Not much of an issue for the temps I usually ride in though.