Your planning to create a backbone?
kung importante yung data tsaka malayu yung location ng mga buildings try wireless and bridge them. Ang problema sa wireless ay may factor ang environment/location tsaka hindi gaano kalaki ang data na madadala nya compared to wired. I suggest using outdoor wireless routers or Point-to-Point antennae kung sobrang layu.
Wired, you need STP outdoor high-grade Cat 6 and Gigabit Switch (managed/unmanaged). Pero kung ako tatanungin mo go for Fiber optics. If you carry large data(video, etc) between buildings pero hindi sila kalayu-an use multi-mode. pero if you want fast and not that large data, tapos malalayu yung mga building, go for single mode.(mas mahal)