I helped a CG do almost this same thing at a couple of buildings we jointly maintain. (He does the computer stuff, I do the phones.)
The interconnection (networking) of the two buildings is over a pair of buried telephone company 100-pair icky-pic cables, end to end, and spliced in an aerial splice case. We used two pairs.
The total cable distance is 360 feet or so. He placed a switch at the distant end to make it work.
I have found that old Cat3 (for instance spare pairs in an abandoned 1A2 25-pair connector cable) works as long as there are no bridged taps.
Hope this helps.
