Here are some online copies of primary resources, other sources, additional context and research (in rough chronological order):
Untangling the William Coxes of early North Carolina: Orange County and surrounding counties (includes 1755 and 1779 Orange County taxpayer lists)
Evidence supporting William Cox in North Carolina in the 1750s-1780s
Thomas Brazier's 1770 will in Orange, Co. (and William Cox's handwritten signature)
Additional records from Greenville County, SC and from related families
Abraham Cox's will and papers, 1857-1860
James Issac Cox & Martha Emiline Cline marriage record, 1876 | 2nd marriage to Eva Foster, 1898
Photos of headstones | Standing Springs Cemetery, Simpsonville, SC (Near Greenville) - Find a grave
Not a primary resource, but some interesting history of the Cox Family in America (no direct connections that I could find, though)