Hello, guy who bought (almost) everything crusade here
You can just run crusade straight out of the
BRB and you codices, and I actually suggest doing that for two or three games to get a grasp of the systems.
BRB missions get boring quick though.
There are two types of books for crusade:
1) War Zone books which contain various amounts of extra narrative rules and a campaign you can play, often with "legendary missions" and "theatres of war"(=environmental effects) you can use for that campaign. Be aware that campaigns implicitly assume that you are playing incursion games, despite never actually spelling that out. The also are quite expensive as they contain... purchasable extra content... for codices.
2) Crusade Packs, which contain missions for all game sizes plus some extra rules like rewards or requisitions for playing multiple missions from that book. All of them contain a full reprint of the core rules as well, which is rather convenient, but you need to keep that in mind when
GW brags about their "112 page mission pack".
In order:
Crusade Mission Pack: Plague Purge - the good thing about this is that it has has some extra agendas, which is helpful to 8th edition codices. Missions are fairly bland though, you are better off with the open war cards deck.
Crusade Mission Pack: Beyond the Veil - Missions are complex, but sometimes don't work that well.
IMO too many are focused on performing actions in the right place. I've also had the experience that the pariah nexus campaign is too complex for beer and prezels gamers, as it requires lots of book keeping. It's a book you should buy when you are looking for new things to do, not when starting out.
Crusade Mission Pack: Amidst the Ashes - Creative missions, similar to legendary missions from the campaign books, lots of unexpected things happen and force people to think outside the box. Extra rules for warp corruption and machine spirits look fun, but so far no one ever "got to it". The good part is that if you don't want to use those, you can just ignore them without any drawback.
Crusade Mission Pack: Containment - Planet Strike only.
IMO a big disappointment, the planet strike rules and missions are horribly balanced, defenders have next to no chance of winning the game as the attacker not only gets a guaranteed first turn, multiple orbital strikes and free deep strikes but also a slew of advantages while the defender gets... a bastion. You cannot play anything but planetstrike with this book so unless you want to spend some effort in balancing it, I suggest to not buy it.
Crusade Mission Pack: Catastrophe - 3 player and 4 player FFA rules only. While there are some cool and creative approaches towards this issue, in the end the games suffer from the same issues as FFA games across all editions have. The game is simply not meant to be played that way and someone will either be winning by sitting on the sidelines or crushed by two players teaming up against them. Great for looting ideas for your own missions, but not really usable out of the box. Honorable/Betrayer mechanic is boring and bad since players have to actively declare units to be honorable or betrayers which only really helps against units which have the opposite alignment.
If you want a crusade pack, I would suggest to go with Amidst the Ashes, it has the most fun/creative missions in it and the Warp Insanity/Machine Spirit rules actually are a flavorful addition for some armies.
All have in common that the "campaigns" they try to support don't really work that well and are strictly inferior to playing the campaigns from the campaign books or even randomly stringing together missions from multiple sources. War Zone Octarius II and Warzone Charadon II are decent books to get started with that (all books have all the rules you need), though the later is a bit more expensive without actually providing anything extra for that money. Since you are playing orks, Octarius 2 is a good buy for you anyways.