I had my '03 XB9R until about 32K miles... it was still going strong, I just didnt ride it. I bought it with just over 20K miles on it. If you keep up the maintenance on them, the motors can last a great while. it depends on how it was treated, and how it will be treated though. with all things, maintenance is going to be required, but Buells are fairly easy to do self maintenance on.
I think the only thing I had to really ever do to my XB9r was replace the stator at about 28K miles...but otherwise, it was a great bike!