Broadleaf evergreens like Chindo viburnum, Ligustrum and Evergreen hollies are more tolerant of being planting in a setting with mature hardwoods and/or pines.  Pines are easier to plant amongst as there is typically less root competition for water and nutrients. Remember that when there is active root competition from mature trees, you will need to provide more supplemental irrigation for the plantings.

It depends on whether you want larger and more instant coverage or can wait for the results as well as the amount of distance you are trying to screen.    Usually, bigger and more instant results will cost considerably more than smaller plantings.

This depends on a lot of factors including the following;  do you want instant screening or are you willing to wait for several years?  Plant type also will determine this as well.  For instance, twelve Green Giants could be planted 8 feet apart in a zig-zag pattern and will form a nice barrier in about 3 years if you have room for them to spread.  If you don’t have room for spread, you may be forced to consider broadleaf evergreens like Emily Bruner Holly, Chindo Viburnum or Ligustrum.  These will not spread as wide so you’ll need more.  A good spacing for the broadleaf evergreens is about 6 feet between each plant so you’d need about 16 plants. 

The most reliable Leyland cypress replacements for North Carolina landscapes are Emily Bruner Holly, Nellie Stevens Holly, Green Giant Arborvitae, and Cryptomeria japonica. All four species provide year-round evergreen coverage and are not subject to the Seiridium canker and Botryosphaeria dieback that have caused widespread Leyland cypress failure across the Southeast.