What is my best possible antenna setup for a 900MHz 24dBm XBee?
I recently used that same Xbee module in a project along with a pair of 6dBi omnidirectional antennas that while not identical look similar to the ones you've found and we got a range of 8 km line of sight but within an urban environment. It may have got further but geography limited how far away we could test and remain line of sight. 

For that test one antenna was mounted out of the window of a house (not elevated above it) and the other one was mounted on a vehicle so roughly the same as your setup. A rough rule of thumb is that adding 6 dB of gain will double the range so you'd expect a pair of 6 dB antennas with a total gain of 12 db to have roughly double the range of the pair of 3 dB antennas with a total of 6 dB. 

Using limited transmit power modules with large antennas does make sense although you can end up exceeding the legal radiated power limits for a band, however if you look at the end of the datasheet there's a list of FCC approved antennas and some exceed that amount of gain. But it's worth looking at that table anyway it might give you a few other antenna part numbers and styles to look at.

About the only other piece of advice I'd give is that for the best range it's worth keeping the antenna cable as short as possible, if you were to use a fairly average coax like RG-58 then 6 meters has about 3 dB of loss at that frequency so for example on the bus it'd be better to place the module right near the antenna and run back the data and power cables if possible.