I don't know about paying off Celtic, but they don't really own Viduka - they bought him on a kind of hire-purchase!!
As for how he'd do in Serie A, he gave Roma a really, really bad headache a couple of seasons ago in the Champions League, and he was linked with them for £15m at the time. He is, as people have said, like Vieiri, but a bit less prolific. He's still a fine striker, who could use a bit of self-dicipline when it comes to training and personal fitness, but who has the skill to succeed anywhere.
Someone said that he has dicipline problems - that's correct. He's gotten himself sent off twice at least this season, and regularly picks up yellow cards for stupid off the ball incidents (reacting to devious defenders for the most part). It's an element of his game that could do with work too, but it's part of his attitude on the pitch. He is lazy, but he's a poacher, so it doesn't matter. How much running does Trez do?