While fiber is a better way to go in many cases, the original question was what kind of cable to run for Cat5 between buildings and if it would receive interference from phone lines.
The ebay equipment is a good idea if you want to save money, however if you are supplying services to a customer, I dont know that they would wait for you to score just the right used equipment on ebay. Generally, my customers want current equipment that I will warranty, and a timely install. The ebay links are for reference only, but they show indoor single mode fiber with SC connectors, multimode trancievers at 10mbs with ST connectors, etc. In short, they are not compatible. While they are an example, it seems more appropriate to give examples of equipment that will actually work together to meet the need.
If you want to do fiber, use outdoor rated product. I suggest transcievers that run at 10/100 mbs or 100 mbs trancievers unless you are running a simple 10mbs network and that is all you need.
Choice depends on so many factors; cost, who's paying for it, desired network speed, desired scalability, etc.
But, to answer the original question again...you could use an outdoor rated, filled Cat5 to achieve connectivity at 100mbs with no active components. You could use outdoor multimode fiber and trancievers to achieve 100 mbs and greater. Neither installation should pose any interference problems.
Good Luck Daron J. Wilson
Telecom Manager
LH Morris Electric, Inc.
daron.wilson@lhmorris.com