Which 50mm lens for the A7 is one of those question where almost everyone will give you different answer. Bellow is the list of the lenses that I would recommend you to consider…
1. Zeiss Planar 50mm f/1.7 C/Y – Very good standard lens with nice coatings, good sharpness across the frame, nice colors. Bokeh can be smoother.
2. Canon FDn 50mm f/1.4 – One of the sharpest lenses in its category. Affordable price, clean sharp images in a good light, not too big or heavy. In the back light it can loose the contrast, so always use the hood.
3. Olympus OM 50mm f/2 Macro (not easy to find for 400 USD though), very sharp lens with unique rendering, one of my legacy favorites. It could be faster and cheaper.
4. Canon FDn 50mm f/1.2 – probably my strongest recommendation for you – except slightly lower contrast, this lens is very close to much more expensive L version.
You’ll find many other preferring Minolta or Pentax, Takumars or Leica. Truth is that all of them are rather good, so it is more about individual preference of the subtle differences in the rendering (character) than about huge differences in the image quality.
You can also consider rather cheap Carl Zeiss Jena Pancolar 50mm/1.8 in M42 mount if you want to start with something good but don’t want to spend too much.