What is the correct answer?

The section with “we will” is incorrect. Should be “we would” The sentence, as written, mixes conditional tenses. Edit: it’s technically ambiguous as to which section contains an error; they simply don’t agree. It could say: “if we are meeting at 6, we will have plenty of time” or “if we were meeting at 6 we would have plenty of time”

The second and third segment don't fit together. If one is correct, the other can't be, but neither is intrinsically correct or incorrect on its own. The most common sentence I can think of that sticks pretty close to this wording and only changes one segment is "If we meet at 6 p.m., we will have plenty of time" (making the second segment in your example incorrect). There are other options too, of course.

The sentence would be correct as: If we were meeting at 6 pm, we would have plenty of time (very unnatural though) OR If we are meeting at 6 pm, we will have plenty of time. As Solid3221 says though, it is much more natural to use: if we meet at 6 pm, we will have plenty of time.