There are lots of f/4.5 Tessar lenses in barrels for quite cheap, i.e. under $100. Not sure what sort of portraits you're aiming for, but something around 180-210mm would be a good general-purpose people-lens or up to 300mm if you have the extension for it (I don't think the Speeds do).
Longer lenses like 300mm are great for tight headshots, but they require nearly twice as much bellows extension as a 180mm. Consider a magnification of M=0.66 for a tight headshot (6x8" at the focal plane):
B = MF + F
So with a 180mm lens, you can get the portrait with 300mm of bellows, but with a 300mm lens, you need 500mm of bellows. Your speed can probably do the former but not the latter.
Last edited by polyglot; 01-27-2013 at 06:19 PM. Click to view previous post history.