I actually know the guys over at RnRVintage and they are good guys, but they have a scary affection on for Guild electrics, particularly S-100s, and have that one priced ~$800 over market IMO. I showed them my '74 S-100c once and they almost knocked me over trying to buy it from me. Made a pretty decent offer too. But no sale here. That guitar is far too sweet.
To the OP. I'd say $600 will get that thing out the door and it's probably worth that. You'll find its worth it to you I bet (but here is not the place for restraint on Guilds).
I too favor buying used, road-worn, beat up "player" axes that I can beat the snot out of without worryand can be found for reasonable coin. My '73 Bluesbird is a great example of that (bought for $700), as is my Tele Deluxe from the same year (traded a strat for it). I personally wouldn't take twice what I paid for them as they are stunning through every amp and my two main gigging axes.
I guess what I'm trying to say here is if you've got the coin, comprehensively test it out with a few amps and if it checks out, offer $600 and ask for some strings etc. with the deal. Then walk away laughing.