400mm Sigma lens for portraits?

I have a Nikon D80 with 85mm f/1.4 and 18-200mm lenses. I'm looking at getting a used 400mm Sigma lens for $400 (CAD) for candid portraits. I find with my other lenses that I either have to get up really close (and it no longer becomes candid) or the portrait is not a close up. I realize the quality won't be as crisp as with my 85mm but I'm not getting the shots I want to get. Any thought about using 400mm Sigma lens in this capacity particularly in low light?

Seems a bit over kill to me. I can get lots of candids with the 18-200 so unless you must be shooting from further than I usually do.
As for low light, with a f/5.6, you'll have to have mighty still subjects.

Wayyyy too long. And heavy.