I did a bit of research on this a while back. A Google search on Nikkor Q 450mm should turn up the threads here and on the LF Q&A plus a couple more.
It seems the 450mm Q is a four-element lens (Q is for Quad) and single coated. It doesn't show up in much of the Nikkor literature and seems to be a predecessor to the multi-coated M and likely the same Tessar design. Since there are relatively few air-to-glass surfaces in a four-element lens, it should be a pretty good performer. The shutter will be older, but as long as it is in good condition, should be serviceable.
I certainly wouldn't pay more for it that an M, however. I got my 450mm M for just over $400. I see them up on eBay for more all the time, but they don't seem to sell at the higher prices. You can likely pick one up for the same price as the few 450mm Qs I've seen advertised there. If you find one significantly cheaper, then jump on it. There is a Fuji L-series in 420mm length that might fit your needs as well. It's also a Tessar design and single coated. They appear for not too much on the used market from time to time. My Fuji L 210mm single-coated Tessar is as contrasty as my multi-coated plasmats.
Best,
Doremus
www.DoremusScudder.com