Wow, I would have thought that the square wave would have been the easy one for a computer.
I agree with your suggestion to just get something out of MCM or another catalog. I got mine that way; I think it was from Korea. It generates sine, triangle, and square waves, and has a coarse attenuator delineated in dB's. Something like $100.
Don't get me wrong, I love vintage gear. But, it's hard to justify sometimes, unless it is a garage sale bargain, and those days are largely in the dim mists of the pre-Ebay era of our culture in my experience.