An interesting line of thought on the assembly of the NT from a liberal scholar, David Trobisch. In short, his thesis is that Paul selected some of his own letters to be published to the churches (cf. "Did Paul Himself Create the First NT Canon"). Some in the early church — notably Marcion — viewed Paul as in conflict with the apostles in Jerusalem (esp. Peter and James) on the matter of grace and works, and the NT itself was assembled and arranged as a collection (in a different order from what we’re used to, with the general epistles by Peter, James, Jude, and John intentionally before Paul’s) partly to refute this disagreement and show that the apostles ultimately represent a united front.
In this regard, he suggests that several books of the NT exhibit a “canon consciousness.” In his article making the case that Polycarp, who was a direct disciple of the apostle John, was the original compiler of the NT during the mid-100s, Trobisch says of Acts:
Like no other book of the New Testament, the book of Acts offers a
view into the whole collection. Being the second volume of Luke’s
work, it provides a link to the Four-Gospel-Book. In its first half,
Acts introduces the authors of the General Letters: Peter, John,
James, and Jude; in the second half, it introduces Paul, the author of
the other New Testament letter collection. In addition, Acts provides
information that makes it possible to identify Luke, the author of the
Gospel, as the doctor who travels with Paul and to identify Mark as
someone close to Peter and Paul.
And in his book on The First Edition of the NT (which has not one but two Amazon reviews apparently by the Anne Rice, and another nice review by someone else named Tom Dykstra that gives a fairly detailed summary of the argument of the book):
When 2 Peter is read as an integrated part of the Canonical Edition of
the Christian Bible, the apparent cross-references to the collection
[of] units are quite astonishing. The Old Testament is quoted
abundantly. Biblical prophecy is explicitly addressed, its relevance
for the present time of readers id demonstrated, and it is related to
a theology of divine inspiration formulated in a manner applied to
other New Testament writers as well. The letter clearly refers to the
canonical Gospel collection by pointing to John (Jn 21), Mark, and the
synoptic account of the Transfiguration. The references to 1 Peter and
Jude serve as links to the Praxapostolos. It presupposes that the
readers have access to a comprehensive collection of Paul’s letters.
In addition to these literary links, the treatment of Peter and Paul
as equals is another trait 2 Peter shares with the editorial interest
of the Canonical Edition. (David Trobisch, The First Edition of the
New Testament, 95).
Conservative NT scholar Michael Bird comments on Trobisch’s argument:
I doubt Trobisch’s main contention that there was a single archtype
“edition” of the NT that became exemplary for later compilations of
the NT writings. Most of the inner-canonical unities that he finds
look like incidental post-compilation observations, rather than
deliberate editorial creations by the formulators of the first New
Testament collection. That said, I think that Trobisch does show how
2 Peter gives us a virtual precis of the NT itself with interwoven OT
themes, references to synoptic material, veneration of Paul’s letter
collection, and incorporation of Jude.
Of course, one needn’t agree with all of Trobisch’s theories (including that Polycarp was the compiler or that the NT includes some forgeries) to find some value in what he says, and it’s nice to see a liberal giving an early date for the finished (or virtually finished) canon. An alternate theory to Trobisch's is, as mentioned above, that Paul himself was the compiler of the first NT canon.
So while it seems that Paul in 1 Timothy was primarily been referring to the OT (and perhaps some subset of the gospels, maybe Luke-Acts) as "the scriptures," it is apparent from his letters he also has a high (shall we say, binding) view of apostolic authority, including his own in whatever medium. As he says in 2 Thess. 2:15: "So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter." If in fact Paul assembled a collection of his own letters for the edification of the church, as suggested above, that would further solidify the collection of authoritative, apostolic documents that would become the NT.
Add to that the "virtual precis of the NT itself" along with the OT citations in 2 Peter, and you have a biblical summary of what was canonical.