If buying private ask the owner to show you where the Exup valve is located on the bike and how often they service it, if you don't like the answers walk away. Someone who has properly looked after their bike will be able to answer your questions and demonstrate that the cover bolts are not seized, this would be a good indication that the rest of the bike has been looked after. If buying from a dealer check out the condition of the Exup cover bolts and if they look rusted and rotten assume the worst, in any case ask that the dealer service the EXUP prior to you collecting the bike.
Check here so you know what you're talking about -
http://www.cartestsoftware.com/fz1/exupmaintenance.html and check out the rest of Pat's page when you have the time as it's all useful stuff.
Like the previous posters have said other than the Exup valve the only real weak points are the OEM rear suspension, corrosion and flaky engine paint. If the bike still has the original shock factor in the cost of getting it refurbished or replaced as this really does effect the bikes handling and ultimately your riding experience.