It would depend on what lens or lenses you already have for the 5D-II. A lot of portraits can be shot with manual focusing lenses. With a $17-$25 lens adapter, you could buy any number of low-priced but superb normal to short telephoto manual focus lenses from e-Bay for as little as $25-$50, and have some fine lenses for the 5D-II.
Pentax M42 thread mount lenses and Olympus OM-series lenses are the handiest lenses. The 5D-II is a fantastic camera body on which to use a 100,105,or 135mm telephoto lens. Some of the older Pentax Super-Takumar lenses are *really excellent* lenses for portraiture, like the Super-Takumar 135mm f/3.5 lens. This lens can be had for $40-$50,and it is a fine lens,even by modern standards.
If you want to shoot low-light, wide-open shots, a Pentax or Olympus 50mm f/1.4 or f/1.8 lens can be bought for $15-$50...with a full frame camera's large viewfinder, it's much easier to focus than it is with a crop-bodied camera with a smaller,squinty viewfinder. The 5D is a very popular camera for the alt. glass crowd that likes to shoot low-light stuff wide-open with superb manual focus lenses that don't cost $1799 or $699 or even $349, but more like $15 to $99.