Good posts by all. I always determine a budget when considering purchasing one of these old pieces of iron. I f you need to replace the exhaust, that's an easy $200 unless you find a used set on eBay or other source, which may go for about that. You need to replace the seat, I auctioned a stock '78 seat on eBay that went for $95. If you replace the "high bars", you'll need all new cables. While the aftermarket switch may be broken, there could also be underlying electrical issues to consider. While you may be able to treat the tank with a rust inhibiter, any crap present may have found it's way into the carbs.
Also, consider the value of the time you'll spend replacing the front brake, etc. While the time I spend wrenching my bike is sort of a zen ritual, my time is very valuable. That being said, how much is that old 550 worth to you?
Good luck
-Bobby