If I move into Sunken Temple with a set without a buff, on conquer can I choose to buff him so I can trigger Sunken Temple?

No, you cannot choose to buff Sett to trigger Sunken Temple in this scenario.

According to RiftJudge FAQ #4146 and FAQ #8537, the "Mighty" condition on Sunken Temple is part of its trigger condition (it appears before the comma). This means the game checks if you have a Mighty unit at the exact moment the conquer occurs.

Here is the breakdown of why this interaction does not work:

1. Trigger Timing: Sunken Temple's ability is a triggered ability that checks for a Mighty unit at the moment you conquer.
2. Mighty Definition: A unit is only considered "Mighty" if it has 5 or more Might.
3. Sett's Trigger: While Sett has a triggered ability that grants him a buff when he conquers, that ability is placed on the chain at the same time as Sunken Temple's trigger. Even if you order the triggers so that Sett's ability resolves first, it is too late; the condition for Sunken Temple to trigger was already checked at the moment of the conquer.

Because Sett was not Mighty at the exact moment the conquer happened, Sunken Temple's ability does not trigger, and you cannot use it to draw a card.