...kz morphed into the ugly as hell, watercooled, belt drive 440 around 1980?...pretty sure they are tough as nails, google up the Suzuki gt380, it's the baby triple, I seem to recall a gs400 in the 80's? Xs400 should be solidly based on over a decade of Yamaha 4 stroke twin technology.
I assume you are thinking of candidates for your $500 challenge...don't forget about the cb360...very dependable, quick little bikes, 6 speed box is a blast...and comfortable if you are not too tall