The original '77 GS550 (along with the '76 GS750 and the '78 Kawasaki KZ650) was the bike that put Honda on notice and sparked the UJM wars of the late '70s and early '80s. It's got a bad reputation for reliability thanks to shortsighted design in the charging system. The regulator/rectifier (combined unit in these) only regulates one phase coming directly off the stator. The other phase direct from the stator and the phase running through the wiring harness are unregulated. This turns the R/R into a giant heat sink, eventually blowing the diodes, and when they blow they often take the stator with them. If you buy it, check the R/R, and if it's stock then replace it immediately with a modern design that regulates all three phases. Apart from that, the engine is a bulletproof, super low-stress design that will run to 100,000+ miles without breaking a sweat.
Is it a
GS550L or a
GS550M? By '82 the L only had a single front disc and rear drum compared to the M's dual disc front and disc rear, but the M has more intricate bodywork and would probably require a tank swap to get that classic look. Both have CV carbs so pod filters would be a cast iron bitch to tune if you want to go that route, but you should be able to find a set of mechanical carbs to fit. My '77 came with Mikuni mechanicals as stock.
As far as aftermarket parts, it's a UJM so all the generic stuff fits, but there isn't much of anything made specifically for it. If you want to buy your cafe parts off the rack you'll want a CB550.
GS550 frame has good bones for this sort of thing. Both the '82
GS550L and
GS550M have a strut at the rear of the frame that imparts rigidity, and the seat and rear fender butt up against the top and bottom of it respectively. Depending on how long you want your seat, the strut is a good place to attach the rear fender, rear edge of the seat (or cafe hump), and brake light. The little outriggers behind that for the turn signals can be cut off without affecting the frame's function. If you want to go shorter (my '77 GS550 measures 25" from the rear of the tank to the rear of the strut) you'll have to weld up some support to replace the strut, like a classic cafe racer rear hoop.
Good luck!
Edit: Almost forgot, you'll want to get real familiar with
thegsresources.com. Those guys know everything about everything about the GS series.
