I was thinking very seriously last fall about picking up a K100... never did. But, here is a link to a VERY good site that covers everything about the three cylinder K75 and the four cylinder K100. It's a big site with lots of info including mods and maintenance. From what I've read, the K75 is actually smoother than its big brother. I'm not real sure about the price, but I know I had no issues considering buying an older well maintained bike. Good luck.
http://flyingbrick.freeyellow.com/