I've never come across bad hinges in 5x0 'books, but certainly have found huge variations in plastic moulding quality across the board. Most 5x0s have really brittle, thin plastic that scratches and snaps very easily, but I've come across one or two that had notably better quality mouldings. Of course, when taking apart your 5x0 be careful with the lower screen "Apple" bezel that almost always snaps tabs no matter how gently you remove it.
I'd love a 550c - it is the ultimate 68K Powerbook by a mile.
JB