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The Greek text behind the NET New Testament is based on the best available manuscripts. These are often, but not always, the oldest manuscripts (MSS). No MS is perfect, just as no translation is perfect. But for the past 120+ years, scholars have increasingly come to embrace the MSS that Westcott and Hort thought were superior. There is no ulterior motive in this; the evidence is incredibly strong that these MSS are very good. Indeed, the more ancient papyri we find, the more ancient patristic commentaries we find, the more WH's opinion is generally vindicated. The original preface of the KJV offered a preemptive strike to those who questioned the translators' judgments in using certain materials. They said that regardless of the source--whether it be heretical or orthodox, Christian or otherwise, all truth was God's truth and thus the data deserved to be looked at and considered on their own merits. KJV followers, unfortunately, usually forget this preface. In other words, the attitude of the KJV translators would certainly not have been to condemn patristic writers' quotations of the New Testament or MSS just because the writers were heretical! Origen, in fact, was one of the most careful biblical scholars of the ancient world, as was Jerome.
The standard vilification of Westcott and Hort usually neglects to mention that there is absolutely no proof that the MSS on which they based their NT were produced by heretics. Indeed, today we know of only one group of MSS that were definitely produced by heretics: several of the Byzantine MSS of the book of Revelation. (These MSS stand squarely behind the KJV of Revelation.) Yet, even these were decent MSS, in spite of the scribes who worked on them.
We have no MSS from the first century as of yet. But we do have as many as a dozen or so fragments from the second century.