I agree with Abacus that many manufacturers do not equip the GM soundset with the best sounds of the respective instrument. But according to my experience, Ketron is an exception, GM sounds have the same quality as the other banks there.
Instead of SD 1 or SD 2, the best small-budget solution would definitely be the SD 1000. I checked the differences to the SD 2 again, there are a number of newer sounds in the SD 1000, also a more differentiated piano sample, and some of them also in the sound bank relevant for GM sounds.