Hello my fellow hobby enthusiasts, with this year coming to a close and a new on beginning, I have decided to start a new 40K project but am seeking some guidance in its creation.

Idea:
A specialized force made by the Inquisition (Ordo Sepulturum to be exact), to fight the arising Zombie Plague around the eye of Terror. The army will have a Bio-Hazard/Rapid Response feel to it.

Army:
For the army I will start off with a Imperial Guard force most likely built around veterans and later on a force of Astartes seconded to the Ordo for the same purpose.

My question to you fine folks is how would you suggest going about building an army like this? I would also like to make it clear that this is going to be a fun thematic army. I am hoping to get a good conversation going to get the creative juices flowing.

Honestly? I'd use Skitarii instead of IG, simply because I feel that the Vanguard would be best suited for what you want.

For Marines, I'd suggest possibly Iron hands, and use a Bike HQ. That way your troops are bikes also, and you can convert them a bit like Kataphron from Cult Mech.

I'd probably also use a lot of Astarte 'Priests'... nothing says plague-cleansing like Santic Daemonology. So either Grey Knights, or maybe a Librarius Conclave.

Interesting idea, I hadn't thought about using mechanicus and Iron Hands would lend a nice feel to the army too.

I was thinking Tempestus Scions with the full masks would be good as well as some Bullgryn with the gas masks. Vostroyan Snipers with gas masks could also be added.
unless you went with all Steel Legion they have gas masks already on them or Death Korps of Krieg. If you wanted to be different and do a little modeling I would use Chaos Cultist they look biohazard ready just a little converting to be more Imperial.
I also like the Skitarii or Mechanicus as troops the vanguard or even the battle servitors could work well in the army as well as the big robots. The forgeworld Tech-thralls would be good troops as well as all the automatons that you could bring. The forgeworld Solar Auxilia troops look interesting and could work really well.

How about your own version of the Deathwatch?

That way you could utilise strengths of different chapters of marines!

I like the idea as a whole, how old you paint them to mark the, as plague fighters so to speak?