For environmental portraits the lens you're using should be fine. For longer there are so many choices in the 180-250 ranges its overwhelming. I have 180/5,6 and 210/5,6 Fujinons but they are quite sharp and may not be what you want. The longer Ektars might work, get one and try, worse case you sell for about what you paid.
I like your Flickr stuff and added you as a contact.