All good info given, read about the different models and know what is what, the
650 club of Australia buyers guide has an excellent description and pictorial on the different models. Take note the US bikes only came out with single disk brakes on the front and the Special 2's 79,80,81, have spoke wheels 19/16 front and rear. Take a copy of the frame and engine no's to be sure you are getting the model as advertised.
You may have to buy a battery as a lot of sellers use that as an excuse for selling...... "A good running bike, just needs a battery", "Just needs the carbs cleaned", "needs a new fuse" be cautious,
If the bike is clean and well maintained, (an honest seller will have some sort of record or receipts), and the owner seems to take pride in his bike it is usually a good sign. Even if the bike is a bit rough cosmetically and the seller has got the bike running it is a good sign.
A lot of XS have had parts swapped between models so what first appears to be.....say a 77 could really be a 74 frame 75 motor with a 77 tank. Go back to the link to the buyers guide and study.
Maybe the best thing would be to buy a bike from one of the members who are selling their bike in the classifieds here.
Check the chain and sprockets for wear, oil leaks, excessively noisy/rattly motor, (can be hard to tell if you don't know your XS's), Take a volt meter and check the voltage,should go up to around 14 volts at around 3500 revs, if not sus rotor, if it climbs to 15 and higher sus regulator and will blow the coils, look to see how clean the tank is, Check that all the switches and lights work, if the indicator lights come on and wont blink sus battery.