The 1977 FX/E Superglide is reasonably reliable for it´s age. But, as with any of these old Harleys´ the reliability depends on the riders relationship with the motorcycle. These things do vibrate, at some speeds a great deal more than others. If you can settle to cruise where it´s smooth (and that doesn´t necessarily mean slow either) you will have less vibration induced trouble. That said, realize the bike is almost 40 years old. All of the oil and fuel hoses should be replaced, brakes rebuilt, electrical systems checked, perhaps a new wiring harness in necessary. All sorts of things should be looked after before you head out. Remember, the middle to late 70´s were HD darkest years, and working for a dealer during those times I can tell you some owners we delighted, some got so frustrated they would try to bring the bike back after 100 miles.
I would suggest you ride a Shovelhead before you buy one, they are very different than the later 80´s vintage Evo (or blockheads.