For the models you've listed you'd need all five sourcebooks, which can be rather expensive, especially if buying all in one go. (I lessened the shock and pain of spending so much money by spreading out the purchases...
)
For Gondor and Rohan, as well as Fiefdoms, Arnor and Numenor you'd need The Kingdoms of Men.
For your Elves and Dwarves you'd need The Free People's. This also includes the Fellowship, Ents and Eagles, etc.
On the Evil side you've got three sourcebooks: Moria & Angmar, Mordor and The Fallen Realms. The first two are self-explanatory, the last contains rules for Isengard, Harad & Umbar, and the Eastern Kingdoms.
Which you buy first is entirely dependent on which factions you play most.
I'd recommend the Free People's book as it contains a wide variety of forces. But then you do mention having quite a lot of Gondor, Rohan etc... the trouble with having such well developed forces and making the transition to warbands is knowing where to start unfortunately
I'll leave the decision to you, as I say it all comes down to which book you'd get the most out of. Best wishes, hope this has helped
EDIT: Beaten by both Bart
and Southy?
oh deary me...