I don't think you get it at all, the Hebrew scriptures give very specific prophesies the Messiah would fulfill.

The scriptures say where the Messiah would be born (Micah 5:2)

The tribe He would come from being Judah (Genesis 49:10)

The lineage He would come from being David's (2 Sam 7:14)

Psalm 22 talks about the method of his death, even the very words that would be used to mock him

Psalm 69 talks about his betrayal by a close friend

To again mention a few...

So the claim that the Messiah is sketched only in Isaiah is not accurate. On Jesus being a king/priest, the book of Hebrews answers you, quoting copiously from the Old Testament. Jesus was not just an itinerant preacher who was executed, He did miracles and rose from the dead! (And if you disagree with the resurrection, it is not enough to just disagree, you have to explain why His original followers as well as people like Paul and His brothers who never initially believed Him started proclaiming that they had seen a resurrected Him).

Christianity was not novel, it was there all along in the Jewish scriptures, Christ preached it during His ministry and then the key thing that convinced His disciples was their seeing Him rise from the dead, after which He told them in Luke 24 that this is what the entirety of scripture from Moses to the writings and the prophets were speaking about all along. That is why the New Testatment writers quote and reference the Hebrew Bible (Old Testament) heavily throughout their writings.

Again my friend, it is up to you. You can do your research into why the first Christians consider Jesus the Christ. There is a reason the Jewish scriptures fit well around him.

An in depth discussion on this is impossible in a comment thread as this is the theme of the whole Bible so I suggest you doing your research into it because if Jesus is who He says He is, there are implications for everybody.