The dealers charge a premium because they (mostly) have expansive knowledge of the Ducati range.
As Neil says in the Bike article, a Ducati is unlike any other UJM (Universal Japanese Machine
) and so needs specially trained mechanics to look after them, hence the higher premium paid.
As for servicing, it MUST be done - no question about it. What Neil is trying to say is that it's better to have a bike which has been looked after properly (ridden and serviced) rather than one that has just sat in a shed. Ducatis do have their faults which appear over time, and a well ridden bike is more likely to have already experienced these faults, leaving it less likely to develop them later ... ie, just after you've bought it!
There's nothing wrong with buying a low-mileage, nearly-new bike, but beware of low-mileage older bikes as these may well need a full service (500+) before even being ridden.
The 748R was a special homogolation model, which Ducati built under SuperSports rules - these state that parts must be fitted to the publically available bike in order for them to be used in the race series. Ducati build a low number of these every year and coat them in the finest components (some people say they even sell them at a small loss) to ensure they can get developed & research changes onto the race bikes.
There's nothing wrong with having a 748R on the road, the engine is fundamentally the same, but they are of a higher-spec both engine and component wise, thus they are more costly to purchase, run and maintain. You'll get the Ohlins kit on an R model, along with extra carbon, so it may well be worth paying this extra premium if it's not too much. The R was also only available in MonoPosto (single-seat) versions.
Having said that, finding one may well be hard enough!
As Lee says, the Grey 748S was a limited edition and looks pretty damn funky. Very different from the red and yellow, but again pretty hard to find compared to the red and yellow models.
As for lockwiring - a lot of people do this to give their bikes the "race" look even if it's never been on the track. Don't be put off by the lock wiring, but ask the seller about it as they may well have an honest explanation.
The availability of parts means it's very easy to buy new sump plugs to replace drilled/lock-wired ones, and race bikes use race plastics, so again these will be pristine. Look for marks on the frame and engine casings (the bits that are too expensive to replace!) as evidence of a bike going down the road.