get a 10" teleoptar
its perfect for what you want to use it for
sometimes they are found in shutters
good luck !
[ edited and added later ]
OP
any lens can be a portrait lens. typically one uses the "normal" lens for the next format as the portrait lens
for the smaller format ... and this is mainly handy if you are just doing head/shoulders portraits or big faces.( or you want a large distance between you and your subject )
the teleoptar i suggested is a telephoto lens so you won't have large bellows draw, usually 10" lenses really
rake out the bellows on a speed or crown graphic ...
what sort of portraits are you hoping to make ?
schneider symmars might be another choice
used as a 2 element lens you might find a 150, 180, or 210 that fits the bill
i have never had trouble with my 210, its heavy, but works like a charm,
and i don't think your camera will have trouble with any of them, unless you
attempt to convert it ( them ) they take a ton of bellows to use converted ...
they are easily found on ebay or used camera dealers ... and not as "rare" as a shutter mounted opera.
http://www.hevanet.com/cperez/test/BigMash210.html
http://www.cameraeccentric.com/html/info/schneider_7.html