[KeefyB]

Hello and welcome.
The main thing to look for when buying a Duke is service history,service history and service history.Have a look at the service/warranty book and find out when it was last serviced and by whom.
The service intervals are,
First service-600 miles or 6 months(whatever comes first)
6000 miles or one year
12,000 or two years (the big one,belts changed,everything checked).
You will need to know what cam belts have been fitted.You need the latest kevlar ones with the red writing on.Also the voltage regulator may be the older type.Its down under the battery and will have "Ducati Energia" or similar stamped on it.If its a large grey one on an alloy plate,your in luck,its one of the later types.There is no red ignition/charging light with this one.
After that its the usual thing of checking for oil leaks,worn/tired suspension,chain/sprockets,tyres,etc.

Good luck!