| 20k in 5 years is fine
I think 4-5 k is average for all bikes (ie not just ducatis!)
I've got a 748SPS with 20k on the clock & it's fine - FDSH - ridden regularly and always got everything done that need to be ( ie not cut corners on servicing)
If it has a FDSH & a wad of receipts it should be fine - but just don't pay too much for it as bikes with 20k on the clock are harder to sell
If you're worried you could get it inspected by an independent ducati mechanic?? cost 50ish
& make sure you get 6-12 months P&L warrenty if from dealer!! | 
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