Bought iron from Joan about the middle of my turns to have some swords in the Bab stack and it cost an arm and a leg and a half.
The Great Library capture is an all or nothing throw. I've seen it work and it is certainly a thrilling turn when the city falls and you withstand the counter attack and all that knowledge is magically poured into your brain. And the fact that is all or nothing removes the "gamey" taste.
As for the GreatLib Exfor, horses, trebs if we can build them, some archers for that free shot on a counter attacks, swords if we have iron. Probably 2x the garrison for bombardment units, horses and swords in equal proportion (40 total might be too many but better to be long offense than short), and a short handful of archers.
Trondheim here we come....