what do you consider "portrait" focal length? -consider that you will have to extend the lens quite a bit for a head + shoulders, and you are looking at only a 2x-4x reduction.. or even 1x for head shot. So a 300mm lens works more like a 400-450mm lens close-up.

I would second the reccomendation of a tessar. If you can handle not having a shutter, a barrel lens would be cheap.
Also look for a gundlach radar.. they go for pretty cheap and are usually in a betax shutter.