The 56 should be a great length and adequately fast. However, as portraits are relatively easy to manually focus, there is a ton of cheap and not so cheap old glass around. I have both the Nikon 55 1.2 and Canon 55 1.2. Both about 230-260 on eBay or FM and commonly for sale. Excellent optics for wide open portraits!
I am considering getting a canon 85 1.2 fd. Yes, big, but that lens is legendary, at least the EF versions are...