Some good comments above. The 640's aren't especially known for electrical problems but the Adv's can have issues with the wires flexing & failing at the back of the ignition switch, a well placed zip-tie or two will prevent that.
Everything you ever wanted to know about 640's: http://www.advrider.com/forums/showthread.php?t=86958
Dunno anything about that particular machine but for $7.5K I'd be wanting it to be in pretty good condition, at around $10K I'd be looking for a 690 instead.
Cheers
Clint
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