Besides the combat patrol, what else does a new player need to start building a Black Templar army and get started with the game?

I posted yesterday in the main 40k sub, but I didn’t get a lot of replies.

I’ve been a casual 40k lore fan for a couple years thanks to some friends, and recently I finally started reading a few of the novels. I’m hooked, and I want to give the game a try.

Black Templars are the faction that resonated with me the most, so I picked up the combat patrol. I know I’ll need to pick up the core rule book, but is there anything else I should grab?

The salesman at the local hobby shop told me I should preorder the Armageddon box that’s coming out soon, but it’s half Orks and I don’t want to drop $300 on something when I can’t use half the minis.

I’m coming from DnD as a hobby, so that’s my tabletop framework. Are there sourcebooks specifically for the Black Templars or anything similar?