Basically, you follow the mould of the 2008 Celtics team (the '86 team too, but, I was too young to talk about that team in depth).
You put a pass first point guard with great defence (Rondo, even though he was still quite raw that year, but, you get what I am going for), pair them with a sniper from 3 point land who can move without the ball effectively (Ray Ray), a player who can get his own shot at any time, and who can score from anywhere (PP), a player who is versatile enough to guard multiple positions, anchors your defence, has a knock down mid-range shot and a post game (KG), as far as the centre position goes, if we are talking about a perfect team, someone like Tyson Chandler would have been amazing on this team, great defender, athletic, will set good picks, and excellent rebounder.
As far as bench goes, someone like a James Posey/Shane Battier type player is extremely valuable, someone with experience, size, and enough versatility to play/guard multiple positions, and can hit the open 3. You'd need a decent enough backup point guard, not sure who to go for here, but, someone like a Mo Williams, who could keep the offence going I guess would be handy, and maybe someone like Brandon Bass as the big Forward backup, knock down shooter, strong inside. That's a great 1-8 man rotation.
I won't go into 8-12, but, you get what I am going for here, as far as building a team that is somewhat realistic.