I certainly would not spend any extra cash to buy the old Mark I 50 1.8 - the optics are identical, the AF motor is the same, and to the best of my limited knowledge a plastic mount and the lack of a distance scale has never yet adversely affected image quality. this lens is so light that a plastic mount is totally irrelevant. Flimsy build, yes, but is there another lens available that could be replaced so cheaply, for less than it's repair cost? Drop or hit any lens, even an L, and the chances are some damage is likely.
The 50 1.4 is a great lens, but if you're not sure about the focal length then go for the 1.8 - it is really sharp, and for the money, unbeatable.