pharoah

For convenience, I'll refer to the author of Luke-Acts in the ensuing discussion as Luke, First, the relevant passages (KJV),

Luke 1
1Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us,

2Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word;

3It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus,

4That thou mightest know the certainty of those things, wherein thou hast been instructed.

Acts 1
1The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach;

Some conclusions I draw from the above:

Luke was aware of many gospels about Jesus before he wrote his account. Furthermore, he claimed to have undertaken his own investigation. This makes the extreme early dates for Luke suspect.

Luke considers the accounts preceding his to be deficient, possibly even erroneous to some degree, which is why he feels compelled to let Theophilus know "the certainty of those things".

From Acts 1:1 and Luke 1:3 we can deduce that Luke believed his gospel to contain the entirety of all of Jesus's sayings and acts, or at least all of the known sayings and acts.

Given the above, the conservative scholars that argue that John preceeded the Synoptics are really shooting themselves in the foot. If Luke was aware of John, he almost certainly dismissed most of it as fictitious.