Look for a Fujinon 250mm f/6.7. It`s a very nice lens, and goes for $4-500 on eBay these days. Very sharp at large apertures. Look (there was a url link here which no longer exists) for an example, shot almost wide open.
I also have a Fujinon 250 F6.7, but I haven't tried it on my 8x10 yet. I did shoot a lot with it in Argentina, and it is an excellent lens, I don't think I can outrun its coverage on my 5x7. It is a very nice focal length for both 5x7 and 8x10. I got mine from Midwest Photo for a reasonable price, but it had some cosmetic issues (nothing serious) so it was about $400 I think. Regardless of which lens you choose in that focal length range, be aware that when shooting wide open, even when focused at the hyperfocal, you will have very decidedly out-of-focus foregrounds. Depending on your subject, that can be a-ok, or it could cause you headaches, as the out-of-focus foregrounds can be distracting. If you have any examples of the kind of shots you want to take with the 8x10, just to give a feel for the subject matter, it would be a big help on advising on the shooting style.