Connecting two buildings, 250 meters, heavy snow country, high lighting risk

Solution 1:

Cat6 for outdoor usage will cost more than a fiber of the same outdoor grade and regardless of lightning protection it will cause trouble sooner or later - we had an abundance of aerial lines of all kinds few years ago so this gave us some insight. Using indoor cable warrants a failure within a year.

You can identify the fiber type (SM or MM) by inspecting cable markings, it's all written on the cable itself; connectors can be identified by photo, they are usually SC/APC (green square), SC/UPC (blue square) or FC (black barrel). If you plan on using existing fiber, you should at the very least check for its continuity using appropriate light source.

For the self-laid fiber the main obstacle is how are you going to terminate it, as you'll need a bunch of equipment and experience to properly weld it. Of course, you can pre-weld splice boxes on the cable somewhere, but the question is how are you going to get these boxes from the street into the building when they are already attached to the cable - that is they can't get through the small neat round hole.

So if I would be you I would have tried to inspect and use existing fiber and if it won't work seek professional help.

Solution 2:

AT&T fiber is single-mode for sure. Multi-mode fiber has very limited reach.

You can get very affordable media converters or SFP modules for 1000BASE-LX which is the protocol of choice here. In case there's only a single fiber you'll need to use 1000BASE-BX(10) which is just slightly more expensive.

Reterminating the abandonded fiber might cost you a few $100 though. You'll want to get SC or LC connectors on the panels.