Good subject, and if we're talking earliest influences, I think we have to start with the films Alien and The Thing ('79 & '82, respectively). Both films had a huge influence on the culture and popularised the themes of parasitism and symbiotics throughout the 80's. Japan was heavily influenced by American culture after the WW2 and the post-war economic boom that had it's height throughout the late 70's, 80's and early 90's.
I believe Baoh, Guyver along with Venom were some of the earliest comic book characters to incorporate this theme, and I dare say they all had some influence on each other since they all appeared '84/'85.
Being a huge "monster" fan, it's also inconceivable that Takaya wouldn't have been influenced by not only Mazinger Z, but also by Go Nagai's other classic, Devilman.
I consider Guyver an awesome hybrid between symbiotic alien anti-hero and giant mecha genres. Throw in some classic Super Sentai and giant Kaiju (like you mentioned, Kamen Rider, but also Ultra-man) and mindless sci-fi body horror like Videodrome and you've got the perfect concoction!
Of course you could always go back further to find roots in Superman/Godzilla, but they were pretty much the precursors to everything.
Nice topic btw!!