The easiest part for the Nets was also the hardest part. P.J. Carlesimo did a good job in difficult circumstances this season, he is a good man, a good coach, someone whose name, by rights, should be included in the modest list of men who’ve won NCAA titles if not for one terrible referee’s call one night in Seattle. But Carlesimo has coached four NBA teams in parts of nine seasons and has never won a playoff series. If a fifth team ever gave him another shot, he would instantly have to go on a 76-game winning streak to reach .500 for his career. It should have been difficult to part ways with P.J. the man, much easier to bid farewell to Carlesimo the coach, and so it is right that the Nets acted so quickly. Now comes the hard part, the part in “The Candidate” where Robert Redford’s freshly elected Sen. Bill McKay asks Peter Boyle, his campaign manager: “What do we do now?” Nets GM Billy King — he’s the one in the Redford role here — said yesterday he isn’t averse to hiring a first-time coach, citing both Erik Spoelstra and Tom Thibodeau as examples of how that strategy can work. Of course, Spoelstra had the gravitas of Pat Riley guarding his foxhole in Miami and it really shouldn’t be interpreted as a shot that having Billy King as your wingman isn’t exactly the same thing. And if King really can identify the next Thibodeau — say, Brian Shaw — then he would be bucking the sheer mathematics that say most fliers on first-timers end badly. The fact is, King and the Nets are in the trickiest position possible, because they need in the worst way to attract an established coach who would be willing to pocket a wealthy owner’s fat paychecks and then swallow hard while facing the future with an inflexible roster already stuffed with players who looked Tin Man-soft across seven games with the lionhearted Bulls. Of course he should contact the Van Gundys, Jeff first, although it’s hard to envision Jeff, who seems as happy and content as can be in his ABC/ESPN broadcasting gig, trading in his currently peaceful lifestyle for his old one as a fitful grinder. Stan? Stan would be a terrific fit, would make his stars accountable and occasionally uncomfortable, and since he’s been to a Finals and they haven’t, they’d have little choice but to listen. Still, there is one ideal candidate King has to investigate, even if there’s only a five or 10-percent chance it could happen, because the more you look at the Nets the more you realize there is really only one man with the credibility, the gravitas, the clout and the capital to take this existing core and teach it how to act — and play — better than they’ve shown as a group so far.