Be careful about the 50/1.8 AI-S. There are three versions:
1) Original. Has the shoe that couples to the metering pin on meter-equipped non-AI cameras. Widely considered to be the best, other than...
2) Japanese pancake lens. Hard to find, and expensive for a 50/1.8. I've never used one, but I read a lot of positive reviews about it.
3) Last version. No meter coupling shoe. It's the same as the Series E lens, but with improved multicoating. It's a good lens, but not as good as the other two.
Most 50/1.8 AI-S lenses on the market are version 3.
I don't think there was a 50/1.8 AI. The slow 50 in the AI world was the 50/2, which is a fantastic lens.