With the exception of mirror lock-up, interchangeable viewfinders, and 100% viewfinder accuracy, the F3a has all the features I need (durability, reliability, interchangeable motor drives, interchangeable view screens, and battery independence).

For portrait work, I suggest you consider the following Nikon lenses:

105mm f/2.5
105mm f/2.8 micro (macro)
70-180mm f/4.5-5.6 ED AF micro zoom
80-200mm f/2.8 AF-S zoom

My personal preference is the 105mm micro.

I did not recommend the 85mm prime lens because it is so close in focal length to what you already have.