Outstanding machines, IMO.
Charging system issues with most of the bigger inline-4 Suzukis of that era, but well documented and easy enough to correct / fix. Might even have been corrected by the OEM by that time, but I can't recall for sure.
Huge support over at
http://www.thegsresources.com/_forum/index.php including manuals, wiring diagrams, parts lists etc.
The ES was the sportier of the two models. The 'E' model was more standard - higher bars and no fairing. DOHC, 4-valve per cylinder heads, screw-and-locknut valve adjusts. Triple disk brakes and Suzuki's early-80s hydraulic anti-dive built into the front suspension. Kinda a gimmick, but effective.
I had the 'tariff-beater' GS700E of '85 which was essentially the same bike but with a smaller bore and slightly hotter exhaust cam (to try and make up for those 50cc). Great bikes!
If the price is right, I'd go for it in a minute with no hesitation!
Kirk