First, if you bury the connection, then it needs to be within a conduit as water and rodents would destroy it rather quickly.
Secondly, you need EXTERNAL wiring and not the more common internal grade coax.
What is your ISP service now; Cable? If so, I would negotiate with the service provider to have a modem installed in the second building, Thus would defeat Print/File sharing but would give more reliability.
If you attempt something like
Code:
isp == modem --- building#1.router - - - building#2 ---
the the link between buildings WILL be ethernet cable and not coax. I would go with cat6 shielded and still place it withing a conduit. The 300 yards is at the hairy edge of the 100meter limit for a end-to-end connection, so you may need a repeater buried in the middle. Run the power on the ethernet
itself using
PoE
You need a repeater and a PoE device may be expensive.
google for "power over ethernet repeater"