Quote: ...but the dof from a 180mm zoom is best is it not?
Define best....... The DOF of a 180mm lens will be shallower than the 50mm 1.4 as lens to subject increases but is that what you want.......or do you want greater depth of field in which case the 50mm is best.......
Portraiture can be performed with any lens as long as you know what you wish to acheive and you can only learn that by practice. Your recent posts give me the impression that you are trying to run before you have learnt how to walk. I strongly advise learning what you can and can't do with the 50mm before buying another lens. This will not only improve your photographic skills but help clarify exactly what you wish to achieve and suggest the kit you may need to acheive it.
Harking back to you 180mm DOF question this picture was taken with a 70-210 f2.8 at somewhere around the 200mm mark and f4...
This picture on the other hand was taken with a200-400 f5.6 at about 300mm and f8...
In both cases the camera has an APS format sensor (Canon) so film equivalent focal lengths would be nearer 320 and 480mm respectively...