Charlie,
Distance from portrayed subject should be at least 1.5m (5ft) in order to avoid "deformation" (that however also depends on the onlooker's perspective, and therefore on the medium - print, screen, projection, etc.). That's just a thumb rule. PLease consider that subjects generally feel more comfortable and behave more naturally when at a longer distance.
By the same thumb rule, a 50mm would frame at least head + upper body (vertical framing) or head + chest (horizontal framing), or maybe slightly less if you cautiously keep the head not too far from the center (once again, to avoid perspective playing nasty tricks), and crop away part of the "overhead" later.
So a 50mm is still OK if "portrait" does not mean mugshot, but upper body shot - Mona Lisa & al.
If you were my neighbour, and not knowing more about your precise intentions beyond your initial post, I'd lend you my Voigtländer 58/1.4 with confidence; these "8 additional millimeters" would be most welcome. The 50/1.8G would be the next best candidate if money and versatility matter, but Andrea is right, it is a bight short.
I can only imagine the "use case" which is not detailed. If it is outdoors (backlight = seashore or similar), stepping back is easy, and a (much) longer FL won't hurt. 180mm-300m or so.
If it is indoors (subject placed near a bright window), stepping back may become difficult, but usually 2.5m-3m would be available. 50mm remain interesting for, say, nude shots, not for traditional head + shoulder portraits - 85mm FL is much more suitable then (on full frame !).
Getting a 85/1.8 is a good idea (they may be cheap), or possibly the old proven 105/2.5, but once again the "flare" issue with backlit subject needs caution. The 105/2.5 is rather good, but not the best here. Results are okay-ish for shooting organs in churches (with windows around and - Gof forbid - behind), but some PP is often required to limit the veiling flare. Those church conditions are however harsh and uncontrollable, so maybe indoors with gaze curtains it would fare better.
I do not know about the flare behaviour of the various 85/1.8. Under more conventional conditions, the old 85/1.8 AF-D is excellent from about f/2.2.
Ah and finally - if you are shooting DX, divide all FLs quoted above by 1.5. In such case, go for the 50/1.8G.
