It's a 3 cylinder smoker, aka two stroke. Two stroke means you're combining oil into the fuel. Some older two strokes had issues with the oil injection system, and many people ran their oil mixture right in the tank and bypassed the old oil injection system. I have no information on whether or not this model was a good example of a good working mixture system. I can tell you, that it will very likely be difficult to find parts for a 3 cylinder pre-GS Suzuki.
Regular 4 stroke, 4 cylinder Suzukis from their GS series of bikes are pretty easy to come by. There are probably compatible parts, but I might take a look through a couple of parts websites to see how easy it is to find parts for this bike. Even though it's complete, it's old. Old stuff needs repaired and replaced, and you would hate to be unprepared for a difficult search. Maybe you're the kind of guy that has the time and money for that type of thing. Mabye you view it as a challenge. Rebuilding a bike is a VERY rewarding hobby, and you most certainly can't go wrong with buying a vintage Suzuki. They are both fun to ride and beautiful, IMHO.
Couple of sites for you to enjoy while you decide....
http://www.suzukicycles.org/ -- database of every Suzuki ever built, with pictures. Yum. It's gearhead porn.
http://crc2onlinecatalog.com/ -- great old bike parts site.
http://www.z1enterprises.com/ -- I think I read somewhere that the guy who owns this site is a Suzuki owner. Parts site.
http://www.thegsresources.com/ -- And of course, the best forum for Suzuki guys in the world. Ask your same question over there. Bound to get some outstanding advice.
Enjoy the journey.
loudest143
82 Suzuki GS850G