I have no experience with the 50 1.7 MD lens.
However, I have bought a couple of cheap M42 and tamron adaptall lenses which I have tried to use with my a580 (using adapters of course) but with unhappy results.
I have a minolta 50 1.7 AF, which I bought through ebay, and I'm extremely satisfied with it. To me, changing from f3.5 in the kit-lens to a f1.8 was the next best thing after buying the slr
To continue with what Thomas said, you'll be using a 1.8 for low light situations, and for a shallow depth of field, and in both cases, i find that the extra help by the camera via AF is extremely helpful. And you always have the MF function available if you want to.